However, a brilliant volley from a tight angle and then a superb free-kick in added time handed Leeds a first win on the road since mid-September.
"We've been trying to find the best position for Ross because if you give him chances he will score," he said.
"I'm delighted for him because when they pulled it back to 2-2 it could have gone either way.
"Is he good enough for the Premier League? Absolutely. But we won't worry about that yet - hopefully he can keep doing that for us.
"It was a good three points for us because we've now got back-to-back wins and go into the international break on a high."
After torrential rain in south-east London, the start was delayed for half-an-hour after a late pitch inspection - but conditions remained tricky for both sides.
McCormack took advantage of a great flick-on from Dexter Blackstock to put Leeds ahead before Stewart's brilliant volleyed equaliser.
For McDermott, the result was particularly satisfying as he saw his side get their promotion push back on track after a run of six successive away defeats.
"I'm trying to build something here and I keep talking about but you have to have the right mentality to win away," he said.
"In most of the games we've been a bit naive but we weren't.
"The most important thing is that the younger players are learning and getting that winning mentality."
- Sports & Recreation
- Cameron Stewart