He's been a regular for the U's - who parted company with manager Herve Renard at the weekend - over the last three years since joining from West Ham, clocking up almost 150 league appearances. He's played 16 times this season, but has been out of the side since the end of October.
City are still thought to be interested in Hartlepool centre-back Michael Nelson as well, but the two clubs are yet to agree a fee.
Elsewhere, left-back Andy Dawson is hopeful of not being suspended for Saturday's game against Tranmere. He's been booked five times which results in an automatic one-match suspension, but wants the last of those five - picked up at Colchester on Saturday for time-wasting - rescinded.