United claimed a hard-fought 1-0 victory over the Hornets at Old Trafford on Wednesday night and the hero of the day for the Red Devils was Mata, who scored a stunning free kick during the 83rd minute of the game.
Now van Gaal has gone on to reveal that he was about to substitute Mata off as he seemed to be struggling with an injury but was more than happy to allow the Spaniard to continue as he went on to net the winning goal.
Van Gaal said: "It was true because he was limping at that time but we were in a better moment so I don't want to change any more.
"I let (Jesse) Lingard do the running again because I had changed the shape, and put (Anthony) Martial in the striker's position and at that time we were in a better shape.
"So then I didn't want to make the change again and then he scored the goal so I am very happy that I let him stay.
"I had wanted to change but then I didn't want to change any more. That was my luck because Mata scored."