Speaking after Saturday's home loss to Barnsley, Nigel Clough admitted two players will be sidelined for the immediate future.
Michael Higdon (hamstring) and Ben Davies (foot) were both absent from the matchday squad against the Reds and afterwards, Clough revealed why.
He said: "Michael Higdon has got a hamstring problem, he felt towards the end of a session at the back end of last week and unfortunately we couldn't get him out there.
"We missed Michael and Marc McNulty will have been disappointed with his contribution when he came on because he didn't have an impact on the game."
On Davies, Clough, who also praised the 'infleunce' of Jay McEveley, added: "Ben will be out for a while with the foot injury he sustained in midweek at MK Dons, but we came through the weekend unscathed.
"Jay (McEveley) has been a great infleunce on the squad since he arrived and he offers us great versatility and balance within the back four.
"He has matured and is still playing with a slight shoulder problem follow the knock at Bradford."
The Blades boss concluded by saying he would like to extend defender Paddy McCarthy's stay at Bramall Lane, after the centre-half played the last game of his initial loan stint with United at the weekend.
Clough finished: "We will have a chat with Paddy and Neil (Warnock) and hopefully we can get something sorted on extending the loan.
"I thought Paddy was excellent again against Barnsley when called upon, he is commanding in the air and has formed a very good partnership with Jay (McEveley)."