McDermott's side were second best at St Mary's as they slumped to a fourth successive defeat ahead of their trip to fellow strugglers Sunderland on Tuesday.
And McDermott was particularly disappointed with his side's second half showing as they lost ground on fellow Premier League newcomers Southampton.
"We are disappointed," the Reading boss said. "We had an opportunity [to win] in the first half and hit the post.
"That first goal was always going to be really, really important. They got it.
"In the second half I didn't really feel we stepped on the way we needed to get three points, never mind one.
"I think it is hard to look and reflect straight after the game. At the moment, you would imagine there would be a lot of doom and gloom around the place after a loss like that.
"I understand that completely because the second half we did not really do ourselves justice, especially with the ball.
"We know we have to be better than that and we now have another game on Tuesday to pick ourselves up."