Eurosport - Peter Leven (PA Photos)
The 29-year-old has been a key member of Oxford's squad this season, playing 25 times and scoring six goals.
He is set to be sidelined for eight weeks due to a keyhole procedure meaning he could return with just a handful of games left in the League Two season.
Manager Chris Wilder described the 29-year-old in November as "arguably the best player in League Two" when fully fit.
Opta bring us the key stats ahead of the More »
Pitchside Europe - 15 hours agoTonight is Germany's time to shine in the More »
Early Doors - 18 hours agoArsene Wenger says Bayern Munich's financial More »
Arsene Wenger - Fri, May 24, 2013 20:44 BSTIf Muhammad Ali was once named the Louisville More »
Pitchside Europe - Fri, May 24, 2013 12:27 BSTJim White says England should pay close attention More »
Jim White - Fri, May 24, 2013 12:10 BST