Paul Paton fired United ahead in only the sixth minute, but Niall McGinn levelled early in the second half and neither side could find a winner.
McNamara said: "We had a good start but over the 90 minutes, I thought it was a fair result.
"In the first 20-25 minutes I thought we had chances to add a second and third goal which didn't come. The longer the half went on, the more Aberdeen came into it and I thought they were better than us in the second half.
"When you've got a goal cushion, you always seem to get deeper and deeper, and we paid the price for that."
The United boss also felt his side had genuine claims for a penalty when Stuart Armstrong went to ground under a challenge from Mark Reynolds in the box.
"It doesn't really matter what I think, but it's one we'll look at later," he added.
"We felt there was contact and that Stuart was pushed."
United remain five points behind the Dons in the race for second, but McNamara believes his team can still contend.
He said "Anything's possible. We have to keep looking after our own performances and results, and wish for the best."
Aberdeen's Derek McInnes was disappointed not to have taken all three points despite watching his side coming from behind for the third time in the space of a week.
He said: "In three league games over the past week, we've come from behind in all three. Regardless of how well we've done in cups, the league is important.
"If we don't finish second it won't be the biggest disappointment. There are some good teams round about us, but we're confident we can keep doing what we've been doing and hopefully we can come out on top."
McInnes was pleased with how his players responded to going behind early, and he singled out Barry Robson for particular praise.
"We didn't start the game as well as we normally do, but we were relentless in the quest to get the equaliser and dominant from there on in," he said.
"If we were a wee bit more clinical, we could have converted that domination into another goal.
"Barry was pivotal. He epitomised everything we've been this season. He took full responsibility, he was aggressive and showed quality at times. His whole demeanour and body language showed he wasn't going to let us lose the game.
"He was in good company with his team-mates, but he was very important to us today and he has been all season."