The young forward is set to face his former club in Monday's Premier League clash but would not have been allowed to leave had the Italian been in charge at the time
The young forward left Eastlands to join Chelsea in 2009 after it was reported that then manager Mark Hughes did not wish to meet his excessive wage demands.
After a loan spell at Bolton, the 22-year-old has now carved out a place for himself in Chelsea's first-team, and has scored a goal in each of his last three Premier League appearances.
But Mancini insists that, had he been boss at the time, he would not have sanctioned the youngster's move.
"It was strange because Sturridge was developed in Manchester [in the academy] and someone let him go," Mancini said, according to The Mail on Sunday.
"For sure he was one of the best young players at City. It was not difficult to see he was a good player.
"If Sturridge was here today, he'd be in the first team. I've watched him many times and know him well.
"When we played Bolton last year [where Sturridge was on loan from Chelsea], I asked him why he had left City.
"He said he didn't know. It would have been better for City to send him on loan to improve than let him leave altogether."
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