Oxford will be without Cameron Brannagan when they take on Pompey at Fratton Park on Saturday.
The former Liverpool midfielder is set to face up to six weeks on the sidelines with a knee injury.
Brannagan was forced off in the first half of Oxford’s win over Rochdale on Saturday and was sent for a scan earlier this week.
Although his injury does not appear a ‘significant’ one, the 21-year is still expected to be out for between three to six weeks.
He missed Oxford’s Carabao Cup fourth round penalties win against league rivals Sunderland on Tuesday.
And Karl Robinson will definitely be without one of his star men of this season when the U’s take on Pompey this weekend.
‘We’re predicting he’ll be back for Ipswich (on November 16), which is great, but he played for 30 minutes on it so the swelling increased around the knee,’ Robinson told the Oxford Mail.
‘So the clarity (on the scan) is not 100 per cent, but it’s not anything significant.
‘You’re talking about anything from three to six weeks, but we think it will be the best case scenario.’