Though currently not on the market, exposure to the Premier League club should sharpen the 18-year-old midfielder's skills, said Afshin Ghotbi, manager of Ishige's J-league side Shimizu S-Pulse.
"If you want to build top-class players, you have to give them various experiences and opportunities to see firsthand what the world level is about," Ghotbi told Kyodo News in an interview published Friday.
"And what better experience for Ishige than to train with the first team of Manchester City, the champions of England, and have (manager Roberto) Mancini see him firsthand and train with some of the top players in the world."
The Sports Nippon tabloid said last month that Manchester City has made a $5 million offer to acquire Ishige.
Ghotbi denied the report, however, saying he has no plan to sell his player and City have not made an offer for Ishige. But many clubs are interested in him, the Shimizu manager added.
"There is interest in him, not just Man City but many clubs in Europe and he deserves it. He's a real fantastic talent," Ghotbi told Kyodo.
"As far as I'm concerned, he's not for sale and we want him to be a part of S-Pulse for the next few years to come," he said.
"There has been no offer, (City) just want to see him and we are doing the same with other players with other clubs."
The high school senior student, who made his professional debut in April, has been a core player in Japan's youth teams with his aggressive playing style and passing skills.