Keane told Sunderland's website: "We're still looking at another three weeks or so. We're doing everything we can; it's just one of those hamstring ones where it can be quite complicated.
"Carlos has seen a specialist. The positive news is that he can keep doing the recovery work that he's doing, we just to be a little more patient.
"Maybe the last time he came back too quickly. We have to be more patient with him."