The Tangerines went down 6-2 to the champions in what was McNamara's first defeat since taking over from departed boss Peter Houston, and saw them slip from sixth in the table to ninth.
United have a quick opportunity to make amends when they visit Fir Park for their rearranged league clash with second-top Motherwell and McNamara is urging them to seize the opportunity.
He said: "I thought Celtic were good on the day but we didn't help with some of our defending.
"But now you've got to look for a reaction and I know they're a good bunch.
"I was disappointed more than anything with how things turned out at Parkhead. We've gone there and scored a couple of goals, and missed a penalty.
"And having watched it again, we get the goal and we're celebrating and they go and hit one up the park and get a corner and score from it.
"It's good that we have this game so soon to try and get that out our system.
"It's a good opportunity to go there and show what we can do, so hopefully we can come back with a positive result."
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- Peter Houston