Mata scored in the 2-0 triumph after winning an early penalty.
"It was important to go into half time 2-0 ahead, but big credit to Reading because they played good football," he said. “They came to Old Trafford with a new manager [Jose Gomes] and played in a very good way. It was difficult to play against them, so I wish them all the best.
"We should have done better, but we are through and I think we are in a moment where, even when we are not playing at our best, we are winning and that is very important to keep that moment, atmosphere and run of results going. We feel now that even if we are not playing very well, we win and we have a clean sheet as well. It is very good for [Sergio] Romero and we are happy."