Henrikh Mkhitaryan is a doubt for the Manchester derby next Saturday after suffering an injury while on international duty with Armenia.
Mkhitaryan, 27, limped off during the 3-0 away defeat against Czech Republic on Wednesday.
The Armenian FA released a statement on Saturday, to Armenian news agency Armenpress, saying that the results of his subsequent tests are "not encouraging."
The statement said: "Mkhitaryan passed a medical examination at one of the clinics in Mlada Boleslav [in Czech Republic]. Unfortunately, the results are not encouraging.
"Usually a week to 10 days is needed for recovery. This means that he will not be able to take part in the Denmark-Armenia match on September 4."
Mkhitaryan later confirmed he had returned to Manchester for treatment and thanked fans for their concern.
He tweeted: "My dear fans, I appreciate much that you worry about me, thank you! I'm now back in Manchester for treatment and hope to be fit very soon!"
My dear fans, I appreciate much that you worry about me, thank you! I'm now back in Manchester for treatment and hope to be fit very soon!
- Henrikh Mkhitaryan (@HenrikhMkh) September 3, 2016
Mkhitaryan, a summer signing from Borussia Dortmund, has come off the bench for United in the Community Shield and in all three of their Premier League matches so far this season but is yet to start a competitive fixture.