The Scotland international, who took his goal tally for the season to 13 in all competitions, opened the scoring against the club which had three bids for him rejected in August.
McDermott said: "We have tried to find a way to play with no wingers and to get Ross up field as much as possible. If he plays right up the pitch he will score goals.
"Every time he plays he looks as if he is going to score. There were bids for him but that is not really relevant because he wanted to stay at Leeds and never wavered."
Middlesbrough had goalkeeper Jason Steele sent off shortly before the interval for bringing down Dexter Blackstock but McDermott added: "Sometimes you are better playing against 11 men. We have found that in the past. It is psychologically harder against 10. We couldn't get that third goal but the result was the most important thing.
"They could have had a man sent off for handball earlier, but I'm glad they didn't go down to nine. It's good to be in the top six but I would rather be even higher. There is much more to come from the team."
McDermott revealed that talks are under way with talented young midfielder Alex Mowatt over a new contract.
Mowatt is growing in stature with every game and McDermott said: "Alex is doing very well for us. Why would he want to go and play anywhere else?"