Roberts and Nmecha both departed City in the summer to Norwich and Wolfsburg respectively but the club opted to terminate both loan deals once the January transfer window opened due to a lack of game time.
Both men have since joined Jonathan Woodgate's side until the end of the season hoping to put a difficult six months behind them
"I played against Lukas throughout the years at Man City," former Manchester United striker Fletcher told TeessideLive. "I played against him at United, and he was on loan at Preston last year as well.
"He's played a lot of games already. All competition is fantastic. It will push me and it will push Lukas.
"Patrick Roberts is someone who will get on the ball and slide the passes through, we've probably been missing that a bit. If we can get them two firing...."