Ole Gunnar Solskjaer accepts he may have to temporarily break up the defensive partnership of Harry Maguire and Victor Lindelof to protect them from injury.
Maguire, 27, became the first Manchester United outfield player since Gary Pallister in 1994-95 to play every minute in a Premier League campaign last season and he has not missed a minute in their 12 league games this term.
Lindelof, 26, has partnered Maguire in all but one league match - the 6-1 trouncing by Tottenham - and only missed three last season.
Solskjaer was unable to name a centre-back on the bench at Sheffield United as Axel Tuanzebe, Eric Bailly and even the forgettable Marcos Rojo are all injured. Phil Jones, also sidelined, was not named in United's Premier League squad.
Although Solskjaer lauded Maguire and Lindelof's partnership, United have sieved 19 league goals in 12 this term and there is a growing clamour for the club to sign a high-quality centre-back next year.
"Victor and Harry have been very consistent," Solskjaer said. "I thought last season as well they were excellent. Harry played the most minutes in European football last season, and I think he’s enjoying playing.
"Victor is maturing, growing and getting more used to the demands of the Premier League. They have been very consistent but they might come to a point where they might have to rest."
Saturday marks the second anniversary of Solskjaer's appointment as United manager, initially on a caretaker basis, though it is a topic he is not comfortable discussing.
"I hate talking about what I’ve done and where we’re at," Solskjaer lamented. "I don’t enjoy that bit. I always look forward and I know there’s a lot of work to be done still to get to the standard and level I want. That’s what we’re working in every single day.
"I’ve really enjoyed these two years, even though games like this make my hair turn even greyer very quickly!"
