Keeper fighting to overcome knee problem.
Sam injured his knee late on in the 1-0 defeat to Coventry on Saturday and limped off at the end of the match at the Ricoh Arena.
Should he not be fit to play, then Chris Lewington will make his league debut for the club, having waited patiently in the wings since his summer switch from Dagenham & Redbridge.
Otherwise, the U's will consider the availability of midfielder David Fox, as he edges closer to fitness after his recent foot injury sustained at Gillingham.
The U's are currently three points from safety but, should results go their way on Tuesday night, they could edge out of the bottom four.
We'll have commentary on U's Player from just before kick-off - click HERE to tune in.