Fellaini missed United's 2-1 Barclays Premier League win at Sunderland due to an injured wrist, with the Old Trafford club subsequently saying on their Facebook site the problem could keep the 25-year-old out for "at least a few weeks'', while manager David Moyes said: "He's going to see a specialist. It doesn't sound too good."
Fellaini has linked up with the Belgium squad, though, ahead of their clashes with Croatia away on Friday and Wales at home next Tuesday, and trained with a protective cast on his wrist.
Belgium coach Marc Wilmots has been quoted as saying that Fellaini has ligament damage and requires surgery but is set to be able to play on Friday.
And reports have also emerged suggesting Fellaini, who joined United from Everton on transfer deadline day, will delay having an operation until December.
On Tuesday afternoon, an article on United's official website said: "Marouane Fellaini has trained with Belgium wearing a protective cast on his injured wrist.
"Although some reports suggest the midfielder faces an operation and a spell on the sidelines, the deadline-day arrival is due to play in a splint against Croatia and Wales in the World Cup qualifiers."
The World Cup qualifying play-offs are in November, although current Group A leaders Belgium look unlikely to feature in them, while United's Champions League Group A campaign finishes on December 10.