Shaun Hutchinson will continue to be assessed by Millwall medical staff after being forced off with a rib injury in Monday's defeat to Fleetwood Town.
The Lions defender crashed heavily to the ground while defending a cross in the first-half at Highbury Stadium and was later replaced.
"Shaun had a whack on his ribs and was struggling to breathe," Lions boss Neil Harris told millwallfc.co.uk.
"He's going to be looked at and we will see how he fares ahead of the Oxford match on Saturday."
The centre-back's substitution paved the way for Jake Cooper to return to backline for the first time this month.
"I am delighted with how Jake Cooper performed at Fleetwood," Harris added.
"He was perhaps unlucky not to have scored a couple of goals late in the day on Monday afternoon."
Winger Shane Ferguson also made a successful return to match action after shaking off a knock to his back, while captain Tony Craig's situation remains uncertain as he recovers from a leg injury suffered in the win over Shrewsbury earlier this month.
"Tony still doesn't have a range of movement in his quad. We are taking it day by day but it has been a frustrating period for him and for all of us.
"Hopefully he will be available soon but we will continue to monitor him."