"At the moment with Smalling and Jones long-term, I'm left with three centre-backs, Vidic, Ferdinand and Evans," said the United boss.
"If I was guaranteed two of those being fit for the rest of the season I'd take that gladly right now.
"It's not been the case in the last two or three years, when we have had a really rough ride.
"I've had to alternate centre-backs. Go back two years ago and we had Carrick, (Darren) Fletcher and (Patrice) Evra there.
"That's a key issue. I can't hide that. I just hope I get a bit of luck in that respect."