Mount had played his part in helping the Reds gain a 1-0 lead, courtesy of Patrick Chinazaekpere Dorgu's fine volley, but did not reappear after the break, with teenager Jack Fletcher taking his place.
As the depleted Reds were already without a number of regular starters for the Boxing Day clash, it was decided that he would not be risked, even if the player was keen to continue.
"He felt something at half-time," Ruben told Sky Sports. "He wanted to go [on] and that is also a good thing.
"He wanted to go to the second half but we cannot lose more players, if you are not 100 per cent.