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 pressure return to the Black Cats and a slide towards the relegation zone, and forthcoming games against QPR and Norwich are not fixtures for a half-fit player.
Then there is the visit of title-chasing Manchester United, and O'Neill is reluctant to pitch Mangane into the uncharted territory.
Martin O'Neill: "He's improving in terms of fitness, and that is good news for us.
"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."