Influential captain Vincent Kompany limped off after 71 minutes against Newcastle with a groin injury and is expected to be out for at least a month.
Fellow central defender Matija Nastasic only returned to training this week after an ankle injury and is doubtful for Sunday's trip to promoted Cardiff.
With Kolo Toure having left for Liverpool in the summer, Pellegrini's options are otherwise limited.
Midfielder Javi Garcia played out the closing minutes against Newcastle alongside Joleon Lescott in a makeshift back line.
With Micah Richards also troubled by a hamstring injury, the only other fit centre-back is youngster Dedryck Boyata, whose City career has stalled in the past two years with loans to Bolton and FC Twente.
If out for a month, Kompany would also miss next week's clash against Hull and the trip to Stoke after the next international break.
He could come back into contention for the derby against Manchester United on September 22.
News of his condition will also be a concern to Belgium, who play an important World Cup qualifier in Scotland next month.
Pellegrini said: "I would prefer the doctor to say the last word, how many weeks, but he is injured I think for at least one month. The doctor can give more information.
"Today or tomorrow we will know exactly what happens with him."
Nastasic was injured in the Barclays Asia Trophy in Hong Kong in July and it was feared he could miss the first month of the season.
But the Serbian has made a quicker than expected recovery and will ease Pellegrini's worries should he prove his fitness.
Pellegrini said: "Nastasic is working normally with the squad and we will see if he starts or not."
The situation with Kompany will heighten speculation Pellegrini may look to sign a new defender before the upcoming transfer deadline.
City have been linked with Real Madrid's Pepe and Atletico Madrid's Martin Demichelis, formerly of Pellegrini's old club Malaga, but Pellegrini would give no definitive clues.
Speaking at a press conference to preview the game at Cardiff, the Chilean said: "It is not dependent on what happens with Vincent Kompany.
"I told you last week that until the last day we will see what we can do, if we can improve our squad or not.
"But we have a very good squad so we will see what happens in the next days until September 2."
The injuries have given Lescott a chance to re-establish himself after falling down the pecking order under Roberto Mancini last season.
The England international is now in the final year of his contract at the Etihad Stadium and his long-term future is unclear.
Pellegrini did not want to disclose information about whether new discussions had been opened.
He said: "We are not talking about those things this week."
Despite the emphatic nature of Monday's win, which has seen City installed as outright title favourites ahead of Chelsea by many bookmakers, Pellegrini is taking nothing for granted against unfancied Cardiff.
He said: "It will be very difficult.
"We know Cardiff are new in the Premier League this year but I think they are a very good team.
"It is the first game they will play at home they will be a very difficult team."
Manchester City Fixtures ...
Sunday August 25
16:00 Cardiff City v Manchester City
Saturday August 31
12:45 Manchester City v Hull City
Saturday September 14
15:00 Stoke City v Manchester City
Sunday September 22
16:00 Manchester City v Man Utd
Saturday September 28
15:00 Aston Villa v Manchester City
Next away game ...
CARDIFF CITY STADIUM
How To Get There By Car & Where To Park
To avoid driving through the centre of Cardiff, leave the M4 at junction 33 and take the A4232 towards Cardiff/Barry. Keep on the A4232 towards Cardiff and then leave the dual carriageway at the B4267 exit. At the end of the slip road, turn left at the roundabout, signposted 'Cardiff International Athletics Stadium'. The stadium is situated a short distance down this road on the right. Please note that parking in the nearby retail park is limited to 90 minutes as is being enforced. Also be sure also to park your car properly, as I have been informed that traffic wardens are out in force on matchdays.
A visiting Scunthorpe fans adds; 'If you pass the ground on your right and then turn right at the next lights (about 250m) and proceed up this road for about 300m, you will see HSS Hire (plant hire) on your right. Turn right in front of this and it leads you into a fenced off area adjacent to Turnstile 7 (the away entrance). You can go straight into the ground without leaving this area. It was also well marshalled on our visit. It is not big enough for teams with big support, but I think it would take around 200 cars. Cost was £5.' Pre-booked coaches get preference for this area, so if your team are bringing a number, then the space for cars may be limited.'
How to get there by coach:
To avoid driving through the centre of Cardiff, leave the M4 at junction 33 and take the A4232 towards Cardiff/Barry/Airport. Keep on the A4232 towards Cardiff and then leave the dual carriageway at the 3rd exit B4267. The 1st exit is the Welsh Museum of Life....2nd exit is Culverhouse Cross. Just before the 3rd turn off you will see the stadium (slightly to your left). Taking the outside lane (of 2) on the slip road, as you approach the lights, you need to be in lane 3 (of 4). You will be bearing round to the right (keeping the pavement on your left) you are taking the 1st turning off this roundabout into Hadfield Road. Travelling approximately 800 meters down Hadfield Road you will then turn left into Bessemer Road. Travelling down Bessemer Road to the traffic lights at its "T" junction with Sloper Rd you turn left. Approximately 400 meters down Sloper road as you pass the bus depot on your right on your left you will see "HSS Plant Hire". Turn Left Here into the protected compound. Thanks to Malcolm a Cardiff Cabbie for these directions.
By Train
The nearest train station is Ninian Park Halt, which is only a five minute walk from the stadium. This station is on a local line (city line-direction Radyr) which is served by trains from Cardiff Central, which run every thirty minutes on Saturday afternoons. On leaving Ninian Park Halt station proceed along Leckwith Road and you will see the new stadium over on your left. Alternatively if you are away fan turn left into Sloper Rd and then turn right into the protected compound at HSS Plant Hire, which leads to the away turnstiles.
Alternatively as Barry Hodges informs me; 'Grangetown station is around a 15-20 minute walk away from the New Stadium and is served by more frequent trains from Cardiff Central, that continue onto Penarth or Barry Island. On leaving Grangetown station turn left, cross the main road and then turn right into Sloper road'. If you are an away fan then turn left into the protected compound at HSS Plant Hire, which leads to the away turnstiles. If for any other turnstile than gate 7, walk on 50 metres, turning left, into the car park towards gate 6 or around the stadium, to the right for lower gate numbers, ticket office or club shop.
If you a bit more time on your hands, then you can walk to the stadium from Cardiff Central, which takes around 25 minutes. As you come out of the entrance to the station, bear left and at the bottom of the station approach road, turn left. Continue straight on into Ninian Park Road, for around a mile passing the Millennium Stadium on your right. At the end of Ninian Park Road turn left and then left into Leckwith Road. You will then shortly be able to see the stadium over on your left (the away turnstiles are on the left hand side of the stadium, as you view it from here). Alternatively a taxi to the stadium from Cardiff Central costs around £7.
Man City Squad Numbers ...
1 Hart, Charles Joseph John
2 Richards, Micah
4 Kompany, Vincent
5 Zabaleta Girod, Pablo Javier
6 Lescott, Joleon Patrick
7 Milner, James Philip
8 Nasri, Samir
9 Negredo Sanchez, Alvaro
10 Dzeko, Edin
11 Sinclair, Scott Andrew
13 Kolarov, Aleksandar
14 Garcia Fernandez, Francisco Javier
15 Navas, Jesus
16 Aguero Del Castillo, Sergio
17 Rodwell, Jack
18 Barry, Gareth
21 Jimenez Silva, David Josue
22 Clichy, Gael
25 Luiz Roza, Fernando
29 Wright, Richard Ian
30 Pantilimon, Costel Fane
33 Nastasic, Matija
35 Jovetic, Stevan
36 Suarez, Denis
38 Boyata, Anga Dedryck
42 Toure, Gnegneri Yaya
43 Tchuimeni-Nimely, Alex
44 Rekik, Kerim
47 Bunn, Harry
48 Lawlor, Ian John
50 Ibrahim, Abdisalam
51 Drury, Adam John
52 Huws, Emyr Wyn
53 Henshall, Alex Charles
54 Kennedy, Kieran
55 Rusnak, Albert
56 Gunn, Angus Fraser James
57 Wabara, Reece
58 Facey, Shay
59 Hutton, Louis Jack
60 Guidetti, John Alberto
62 Razak, Abdul
63 Johansen, Eirik Holmen
64 Mesquita Lopes, Marcos Paulo
65 Ntcham, Jules Olivier
66 Fofana, Seko Mohamed
67 Bossaerts, Mathias
68 Leigh, Greg
69 Swan, George
70 Evans, George
71 Denayer, Jason
73 Glendon, George
74 Plummer, Ellis Kane
75 Bytyqi, Sinan
76 Mayifuila, Jordy Hiwula
77 Cole, Devante
78 Pozo La Rosa, Jose Angel