The Senegal defender is yet to feature for the Black Cats after arriving in January from Al Hilal.
O'Neill said: "He will play for us, otherwise what's the point in bringing him here?.
"Kyriagkos last season came in and we knew he had not played for some time.
"He said he'd done quite a lot of running with his German team so we threw him in rather quickly and he didn't get off to the best start and lost a bit of confidence over it.
"Whereas when Mangane came over here he looked a little short of fitness and that's not casting aspersions on another club.
"I hate it when managers do that, it's a great old excuse.
"He was just a wee bit short of match work but he's gaining it now so I'd like him when he steps into the side to be as well prepared as possible."