The holders launched their defence of the title in the third round by routing their Championship opponents 7-0, with all the goals coming in a frantic second half.
Lampard scored the first and last and put in a man-of-the-match performance three days after coming off the bench to score an emotional equaliser against former club Chelsea.
Pellegrini is spoiled for choice in midfield, but the City boss said: " For me it's never difficult to put a good player in the team
The problem is when you don't have the players
"I think it was very important for Frank to score today again
He's a very important player
I said after the game against Chelsea that he's an important player for our team
"Maybe he's now 36 years old and he can't play three games in a row but he's a top player."
Lampard made the surprise move to join City on loan last month from sister club New York City having been released by Chelsea in the summer
The deal is until the start of January but, with the Major League Soccer season not beginning until March, Lampard's form has led to speculation that Pellegrini will try to keep him for longer
The Chilean said: " At the moment I think it's not the most important thing
We'll see what happens in January
"We have two or three months more to decide but I will look at what is important for the club and for the player
We must talk about a lot of things but it's not impossible."
Pellegrini picked a very strong team but City were frustrated by a resolute Wednesday display in the first half
However, once Lampard bundled home the opener two minutes after the break, the floodgates opened
Edin Dzeko scored in the 53rd minute and Jesus Navas made it three just over a minute later
On the hour mark Yaya Toure slotted away a penalty after Kamil Zayatte saw red for felling Lampard and Dzeko added his second 13 minutes from time - his 11th in 11 League Cup games
Teenage striker Jose Angel Pozo marked his debut with a goal in the 88th minute and Lampard slid in the seventh in injury time to set up a fourth-round meeting with Newcastle
The result ended a run of four games without a win, and Pellegrini was particularly happy with the manner in which his team played in the second half
"Maybe it was the change of attitude in the second half," he said
"I think we were more aggressive.
"When Sheffield played with one player less, of course it was easier for us, but when we scored the fourth goal, I think that is the way this team must play and the best reward is playing that way, more than the amount of goals that we scored."
Wednesday crossed the Pennines full of confidence after only one defeat in their opening 10 games of the season
Manager Stuart Gray tried to take the positives from the first half but conceded the result was hard to take
He said: "We had a nine-minute mad period where they scored three goals
It just shows the gulf between the Championship and a top-four Premier League team
"They're world-class players and every mistake we made in that nine-minute period we got punished
I think two of the goals we were probably in their half and gave a sloppy pass away
"After the sending off it was damage limitation
You take confidence from the first 45 minutes because I thought we dominated possession
"Gary Madine had a reasonable half-chance, and it's about taking your chances
We've had seven clean sheets this season
To lose 7-0 has maybe shaken us a little bit."
Source : PA