The 27-year-old appeared to skid a little on the wet surface as he fired the ball in and he conceded afterwards that he had been "a bit fortunate".
Asked if he had had any doubts about taking the penalty, Rooney said: "No, not at all. Obviously I was disappointed at the weekend, not only to miss but to miss the target.
"I said after the game it wasn't good enough and (against Braga) I was a bit fortunate; I slipped but thankfully it's gone in so I'm happy with that.
"At the time it happened so quick, but obviously I've seen it going towards the goal and the keeper going the other way.
"I was happy with it going in but obviously I've been quite lucky with that."