Blades manager Kevin Blackwell said: "We're waiting for the specialist to re-read the scans because one shows no damage and the other indicates that there might be a slight tear.
"The experts will re-do them and then compare to come to a decision. Fingers crossed it's just a strain and not a ligament tear because if it is that will be the end of him until next August.
"Even if it's not he must be doubtful for Saturday."
Naysmith, a £1million signing from Everton in July 2007, has been a virtual ever-present for the Blades this season.