The striker damaged a cruciate knee ligament in Saturday's 2-0 defeat to Bristol Rovers and although surgery is not needed he is still likely to be missing for three months.
The Burton loanee had made five appearances without scoring a goal before the injury struck.
Alfie Potter replaced Richards in the defeat to Fleetwood on Tuesday and he could be given the nod from the start again as the U's look to get back to winning ways.
Deane Smalley came through his return to action for the first time since October unscathed.
United are now just nine points clear of the relegation zone having only won twice in 2013.