Ole Gunnar Solskjaer confirmed Marcus Rashford is missing against Liverpool due to the back injury he sustained against Wolves last week.
Rashford came on as a second-half substitute in the FA Cup third-round replay but had to be replaced just 16 minutes after coming on.
"The knock he got against Wolves was bad enough to keep him out," Solskjaer told MUTV. "He's always recovered quickly before but this time it takes longer, so we just have to keep monitoring him, do another scan over the weekend and see how he is."
In a separate pre-match interview with Sky Sports, Solskjaer said he does not expect Rashford to be available until after the February winter break, which ends with United's trip to Chelsea on February 17.
"I wouldn’t expect him to be back in the next few weeks," Solskjaer said. "We have a mid-season break and I’m not sure we are going to see him before then, no."
Luke Shaw starts for United at Anfield and Solskjaer said the left-back is ready to play having not featured for the Reds' since the New Year's Day defeat by Arsenal.
"Luke's ready for this, yeah, definitely," United's manager added. "We've had Brandon [Williams] playing left-back and he's done really well. Today Luke is back in for us, he's been ill more so than injured, so he won't have any problems."
Shaw looks set to be deployed as a third centre-back at Anfield alongside Victor Lindelof and Harry Maguire, with Aaron Wan-Bissaka and Brandon Williams occupying the right and left-back berths respectively.
