Ki has not played since the 1-0 home defeat by Everton earlier this month and has missed the last three games which have seen Sunderland pick up seven points to move out of the relegation zone on goal difference.
The South Korea international is suffering from tendonitis and Sunderland boss Gus Poyet is unsure if Ki will feature again before the end of his season-long loan.
"Ki has been to see a specialist in London who has confirmed that it is tendonitis of the knee and needs to be treated carefully," Poyet told the Sunderland Echo.
"It's not something that has come on suddenly, he has had a problem with it for a while.
"He had been playing through it but it slowly got worse and now I really don't know if it will be possible for him to play again for us.
"We will hope for the best and we will see.
"We have to be careful with him. If it was a more straightforward injury like an ankle for example we might take a risk, have an injection or whatever.
"But when it is muscle tissue like the tendons, you are always wary.
"So we will leave it to him. If he feels ready at any stage, we will involve him."
Sunderland travel to Manchester United on Saturday before rounding off their season with home games against West Brom and Ki's parent club Swansea.