Steve Bruce admits he wants to bring John Mensah to the club on a permanent basis but will only do so if he can prove his fitness.
The 28-year-old has spent the last two seasons at the Stadium of Light on loan from Lyon but has been unable to get a run in the side because of a chronic back problem.
Bruce is set to run the rule over the Ghana international in the summer but admits he has a difficult decision on his hands.
"We have a decision to make on John at some stage and I can tell right now, it's not going to be an easy one," he said in The Journal.
"He's one of the best defenders I've ever seen when he's fit, but he's often not and you have to decide on where the balance stands in terms of the club getting value for money.
"When he's fit, you have to put him out there. I've given up trying to analyse it. He played all the World Cup, didn't he? Day after day.
"He's a smashing lad and a wonderful player, an excellent team player, and a good influence on the dressing room - in the end I felt it was worth taking a risk because he's such a quality player."
Mensah has made 11 appearances for Sunderland this season and last featured against Tottenham earlier in the month.