The Senegalese defender has not played since his arrival from Saudi Arabian side Al-Hilal in January, but has made the bench as well as turning out for the club's Under 21 side in their recent encounter with Everton.
A run of five games without a win has seen the Black Cats slide towards the relegation zone, but forthcoming games against QPR and Norwich offer a chance to head back in the opposite direction.
However, with these fixtures followed by the visit of title-chasing Manchester United, O'Neill is reluctant to pitch Mangane into the uncharted territory of the Premier League before he is certain the international is fit enough.
"He's improving in terms of fitness, and that is good news for us," O'Neill said.
"If we had a few more points on the board, I think then you could risk throwing him into a game.
"At some stage or another, he will play because we want to find out exactly what he is capable of doing.
"He will just have to bide his time, but I would like him to be as fit as he possibly could be when he gets that opportunity."