The 33-year-old striker has been on a month's loan at Bramall Lane but now his days in a red and white shirt look over after he was injured in the Blades win against Stockport on Saturday.
"Paul is having a scan and we'll have to see what that shows up," said Blades boss Neil Warnock.
"It doesn't look as bad as we first thought but I think it's fair to say it looks like he's played his last game for us.
"He'll know more about how long he's going to be out for though once we've seen the results of the scan."