Jason Puncheon's solitary goal was enough to secure victory for Saints, who climb to 15th in the Barclays Premier League standings, while the Royals remain in 19th.
"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."
Reading now face a trip to Sunderland on Tuesday night and McDermott added: "We know we have to be better than that and we now have another game on Tuesday to pick ourselves up."
- Sports & Recreation
- Barclays Premier League
- Jason Puncheon