Sturridge has established himself one of the first names on the teamsheet at Chelsea this season, scoring nine goals in 20 Premier League appearances - and his form has earned him England recognition.
He joined Chelsea nearly three years ago when his contract at City expired, with the Manchester outfit unwilling to meet his reported wage demands of £65,000 a week. And Mancini, who took charge at City some six months later, has queried the wisdom of letting the forward move to a rival side.
"I asked him last year when he played [on loan] for Bolton why he left and he said he didn't know," Mancini said, according to the Times. "Given the chance, I think I would have liked to have kept him, but now he is at Chelsea, we can do nothing.
"I know for sure he was one of the best young players that was here. It was not difficult to see that Sturridge was going to be a good player. It was strange because he started out in Manchester and someone let him out of the club."