Mainwaring suffered a broken wrist in training on Wednesday having only recently made his comeback from a broken leg and will miss the rest of the season.
Defender Mullins has missed the last two games with a back injury and will face a late fitness test to determine whether he will be available for selection.
Greg Tansey will sit out the last three games of the campaign as he continues to recover from a torn thigh muscle.
The relegated Hatters have only picked up two points from their last 11 games and are looking for their first win since the 4-3 victory over Wycombe at the end of February.
