Rio Ferdinand will miss Middlesbrough’s visit to Old Trafford on Monday after suffering a back spasm in the warm-up at the Britannia Stadium.
The centre-half was named in Sir Alex’s starting XI but was forced to withdraw shortly before kick-off.
Now Ferdinand is expected to miss the Reds’ next two fixtures.
“Rio won’t play against Middlesbrough,” Sir Alex confirmed after the match.
“He had a back spasm again. We have to get to the root of this; he’s had this back problem too many times, so we want to get to the bottom of it.
“First of all, we’ll give him complete rest, but I think he’ll be okay in about 10 days.”
United's FA Cup third round trip to Southampton on 4 January could come too soon for Rio, although he could return in time for the Carling Cup semi-final first leg against Derby County on 7 January. The Reds welcome Chelsea to Old Trafford on Sunday 11 January.