The 24-year-old departed the Etihad Stadium and the Champions of England in the summer in search of first team football for the Black Cats and faces his former employers again
The Italian admitted that the England international had grown frustrated with his lack of first team opportunities at City and now faces the man he sold to Sunderland for £10 million in the summer for the second time since his departure.
"I didn't say to him to leave. Adam said he wanted to always play and for that reason I let him go," Mancini told reporters.
The 24-year-old has made a quiet start to life at the Stadium of Light, scoring two goals in 15 appearances in the league this season and has seen his international opportunities dwindle as a consequence.
"For me Adam was an important player but I understand the situation," Mancini continued.
"He is young, he wants to play every game."
Mancini's side are second in the league heading into the Boxing Day fixtures, four points behind leaders Manchester United having gained two points on them after their rivals' 1-1 draw at Swansea on Sunday.