Manchester United could be without right-back Rafael da Silva for up to five weeks after he suffered a hamstring injury in the Community Shield.
The Brazil international only played the first 16 minutes of the 2-0 win over Wigan at Wembley before he was replaced by Chris Smalling.
And he is likely to sit out the Premier League clashes with Chelsea and Liverpool and could face a battle to be fit for the Manchester derby on September 22.
“[It is] a hamstring, I could be out four, five weeks,” Rafael told the Guardian while manager David Moyes downplayed his removal.
“Rafa said that he felt his hamstring and in a game like this when you have an opportunity to make substitutes, it was fine,” he said on MUTV.
Rafael was already certain to miss Saturday’s trip to Swansea when he serves the final match of a three-game ban he received for his sending-off against Chelsea in May.
His twin brother, Fabio, is one option to replace him along with Smalling and Phil Jones, who moved to right-back in the reshuffle when the Brazilian went off at Wembley.
Moyes’ only signing so far is a right-back but the young Uruguayan Guillermo Varela was expected to be loaned out before Rafael was injured.
Rafael was one of United’s most impressive players last season, scoring three goals in 37 appearances.