Marouane Fellaini is struggling to prove his fitness to David Moyes ahead of Manchester United's clash with Arsenal after suffering another setback on his road to recovery, Goal understands.
The Belgian - who has not played since October because of wrist, back and groin problems - suffered a hamstring strain in training last week which has put his participation against the Gunners in doubt.
Fellaini had returned to first-team training along with Robin van Persie but while the Dutchman is now back on the pitch, United officials fear Fellaini may be out for another two weeks.
Fitness issues and poor performances have dogged the 26-year-old since his 27.5-million-pound move from Everton on the final day of the summer transfer window.
Another concern for Moyes is the fitness of Jonny Evans, who is still struggling with a calf injury he picked up in United's 2-1 defeat to Stoke City earlier this month.
In his absence, Nemanja Vidic and Chris Smalling put in a less than convincing performance as the Red Devils slumped to a 2-2 draw at home to Fulham on Sunday.
Wednesday's clash with Arsenal promises to be a crucial clash for both sides after Arsene Wenger's men were humiliated 5-1 by Liverpool at Anfield on Saturday.
But while the Gunners remain in the title race, United lies seventh, nine points behind Brendan Rodgers' men in fourth as they fight to ensure their participation in the Champions League next season.