Oxford are waiting to learn the extent of key midfielder Cameron Brannagan’s knee injury ahead of their trip to Pompey on Saturday.
The former Liverpool man was forced off in the first half of the U’s’ 3-0 win over Rochdale at the Kassam Stadium.
Brannagan, 21, was set for a scan yesterday, with Oxford hoping his knock is not as bad as feared.
The midfielder has played a key role in Karl Robinson’s side’s storming start to the season.
He has featured in all 15 of Oxford’s League One matches this term, netting five goals and providing five assists.
Robinson rates Brannagan as the best player he’s seen in the division so far this campaign.
He admitted losing to injury would be a ‘massive blow’.
Robinson told banburycake.co.uk: ‘Cameron Brannagan will have a scan this afternoon, he's still in severe pain this morning and can't put any weight on it.
‘We'll have to wait and see for the true reality of the injury to come.
‘While we're in this moment we just hope it's not a bad one - for him most importantly because he's probably been the best player in League One this season, from what I've seen.
‘That (would be) a massive blow to us.’
Oxford welcome Sunderland in the Carabao Cup fourth round this evening prior to making the trip to Pompey in League One on Saturday.