A goal by Ivan Sproule secured three points for County and left United, who had Barry Douglas sent off, down in sixth spot.
McNamara said: "I think that is it over for Europe. Realistically, it would be very difficult for us.
"I thought it was a game we had to win and I'm very disappointed with the way it panned out, especially with the sending-off.
Douglas saw red 11 minutes after County took the lead, but McNamara struggled to see anything wrong with his player's challenge on Sproule.
"The first goal is always crucial when you come here, the way they play and defend. It's difficult," he added.
"I've just seen Barry's challenge on the monitor there and he has won the ball. I didn't even think it was a yellow, never mind a red."
- Sports & Recreation
- Ivan Sproule