The lead lasted barely two minutes as Iago Aspas' deflected effort wrong-footed Diego Lopez to bring Celta level.
However, for all his good work in the first-half, Varas was at fault for the winner as he clipped Kaka inside the area and Ronaldo stepped up to smash home the penalty for his 42nd goal of the season. Jose Mourinho again rested a number of first-team regulars as he made seven changes from the side that defeated Manchester United in midweek.