The Royals went top of the npower Championship at the expense of the hosts after Adam Le Fondre came off the bench to fire his side to a 3-1 victory at St Mary's.
"I thought tonight was an incredible night," boss McDermott said. "They are a good side Southampton, especially at this place. They don't lose too many games here."
He added: "We had to hang in once they got back to 1-1 and had a 20-minute spell where we had to hang on.
"We made a substitution that was positive in winning the game and that is what Alf (Le Fondre) does. He is born to score goals. He got two chances and scored two goals."
McDermott admitted he had "no idea" how the automatic promotion chase would pan out, but counterpart Nigel Adkins remained resolute in his belief that Southampton would be playing in the top flight next season.
"It was a good game of football, there was a good atmosphere," he said. "From our point of view, we believe we could've done better with the three goals that we've conceded. From Reading's point of view they will believe they were very good goals because they were good finishes.
"Once we equalised there was the belief there that we could have gone on to win the game, but we haven't done.
"The principles and how we want to play it is good, but it's about winning football matches and tonight we haven't got that.
"It is in our own hands. Control the controllable - we keep saying it and it is very much what we do. All we can do is concentrate on what we do hence not going to watch West Ham or anything like that because we can't control that."