Wayne Rooney's delaying tactics over a new contract have sparked fresh rumours that he could leave Old Trafford in the summer.
It's understood that Rooney is avoiding negotiations over a contract extension until after the World Cup, sparking concerns that his decision on whether to stay could hinge on Champions League qualification, with United currently struggling in seventh place in the Premier League.
"We would have to wait and see what [would] happen if that situation arose," said United manager David Moyes.
"But if you looked at his performances you'd have to say he's certainly playing at his best."
Moyes has confirmed Wayne Rooney, who has 18 months on his current deal, will miss Sunday's FA Cup third-round tie against Swansea.
Rooney has been nursing a groin problem and sat out the trip to Norwich on December 28.
The 28-year-old did return for the New Year's Day home defeat by Tottenham, but was pictured on Friday leaving a private hospital in Manchester having had further scans to establish the extent of the injury.
United have given no update on Rooney's fitness.
However, with a trip to Sunderland for the Capital One Cup semi-final first-leg looming just 48 hours after the Swansea encounter, Moyes has already decided Rooney will not face Michael Laudrup's men.
"He's got a groin injury and won't be available on Sunday," Moyes told reporters.
"He's got a chance [of facing Sunderland]."