(CNN) -- Portuguese winger Nani will almost certainly miss the start of the English Premier League season after picking up a shoulder injury in Manchester United's Community Shield defeat to Chelsea.
Nani clutches his shoulder after going down under a challenge from John Terry at Wembley.
Nani opened the scoring for United and impressed on the left wing before being forced off in the second half at Wembley after falling awkwardly following a tackle from Chelsea captain John Terry.
Nani is having a scan on Monday to discover the full extent of the injury, which may require surgery.
"I hope it's not a long-term injury. But he has done some damage to his shoulder," manager Alex Ferguson was quoted on the club's official Web site www.manutd.com.
Nani has flown to join the Portugal squad for the match against Liechtenstein on Wednesday, but is a doubt for that fixture and United's home opener against Birmingham on Sunday.
"We could not put the shoulder back in, so that is the worrying part for us," Ferguson added. "He had done very well and we were very pleased with his performance."
United hope Nani will step up to the plate and replace the departed Cristiano Ronaldo as their major threat from wide positions this season so his injury comes at an inopportune moment.
He is added to a list that also includes Nemanja Vidic (calf), Wes Brown (thigh), Gary Neville (groin) Edwin van der Sar, who has a broken finger.
The absence of van der Sar will be most keenly felt, with stand-in Ben Foster enduring a difficult afternoon against Chelsea as United lost 4-1 on penalties after a 2-2 draw in regulation play.
Foster, who is in the England party for Wednesday's friendly against the Netherlands, saw Frank Lampard's second for Chelsea slip through his gloves and punched poorly for Ricardo Carvalho's opener.
Ferguson must now choose between Foster and Tomasz Kuzsczak for the opener against Birmingham.
"Knowing Ben and the ability which he has, he should have done better with the second goal," said Ferguson. "However, it was his first big challenge.
"He has waited a long time, has had his injuries, and in pre-season I spread the load between the three 'keepers, and maybe we did not have enough minutes for him."