(adds manager's quotes)
MANCHESTER, England, Dec 23 (Reuters) - Manchester United's big-money signing Marouane Fellaini faces several weeks on the sidelines after having an operation on his wrist, manager David Moyes said on Monday.
Moyes, who was also Fellaini's boss at Everton before both moved to Old Trafford before the start of the season, said at a news conference the 26-year-old was unlikely to play again before the end of January.
"We think probably he will be out for six weeks but there's a chance it could be shorter than that," he told reporters.
Fellaini has also struggled with a back problem this season and Moyes said: "His back problem was just going to take time. He has a muscle injury in his back which wasn't healing.
"That is maybe going to take longer than we expected so that's when we decided to get the wrist done. He's been unlucky.
"He does not feel 100 percent fit because of his wrist so he's found it difficult."
The two injuries mean the mop-haired Belgium midfielder could miss both legs of United's Champions League last 16 clash with Olympiakos Piraeus in February and March.
Fellaini has struggled this season to replicate the form that persuaded United to pay Everton 24 million pounds ($39.25 million) for him in September.
He has yet to score a goal in 11 appearances for the Premier League champions.
Moyes, though, gave an enigmatic reply to a question about whether striker Robin van Persie could make a "miracle" return during the Christmas programme after being out of action since Dec.10 with a thigh problem.
"Well there are always miracles at Christmas time, so let's hope there will be one for Robin and (the injured) Michael Carrick.
"That's when miracles happen, so you just never know," a beaming Moyes said.
The Old Trafford club, who are down in eighth place in the table, travel to former United defender Steve Bruce's Hull City side on Dec. 26 before playing at Norwich City two days later.
The third match of their holiday programme is against Tottenham Hotspur at Old Trafford on New Year's Day.
($1 = 0.6114 British pounds) (Writing by Tony Jimenez and Mike Collett,; editing by Pritha Sarkar)