The 29-year-old defender signed from Al-Hilal is a Senegal international and should add experience to the Sunderland back line. However, Black Cats boss O'Neill thinks Mangane's impact will be limited until his fitness reaches its optimum level.
"He's got a great physique and presence and certainly has what it takes to do the job," O'Neill told the Daily Star.
"But it's fair to say that Kader's a little bit behind the rest of the lads in terms of fitness.
"Once he's fully fit, he'll be pushing for a start in a position where we've been really stretched of late."
"Whenever a loan signing comes in from abroad, part of the deal is that there has to be an option at the end of the deal for a permanent signing and that's the case with Kader."