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."
Despite the loss against his former club, McNamara maintains they can still take some positives into the clash with Stuart McCall's high-fliers.
He said: "We created a few chances at a difficult venue and we scored a couple of goals.
"It gives me a lot of encouragement that we've got lads there who can go and hurt teams, as we know, but we've got to concentrate on the defensive side as well and make sure we do our job better in that respect.
"We know there are things there that defensively we need to work on.
"It's fair enough if you are getting carved open or guys with ability beating your defenders, but not just scoring a header from a corner.
"They are cheap goals to lose so that's what disappoints me most."
McNamara expects Willo Flood to be in better shape at Fir Park, having played against Celtic while still suffering from flu symptoms.
McNamara said: "Willo had been ill during the week and you could see he was struggling a bit.
"He hadn't eaten a lot so we knew it was a wee gamble to play at Celtic."
McNamara expects to have a fully fit Willo Flood for the Clydesdale Bank Premier League clash with Motherwell.
Flood came off in the first half of Saturday's 6-2 defeat at Celtic Park while suffering from the remnants of a flu bug.
United will be without Gavin Gunning after the defender suffered a knee injury that could finish his season.
Meanwhile, Motherwell defender Simon Ramsden is fit for the Clydesdale Bank Premier League clash with Dundee United.
Ramsden went off against Inverness on Saturday with a suspected broken hand, but X-rays proved there was no fracture.
Motherwell are still without the suspended Keith Lasley and Fraser Kerr.
Provisional Motherwell squad: Randolph, Saunders, Ramsden, Hutchinson, Hammell, Francis-Angol, Hateley, Law, Higdon, Ojamaa, Humphrey, McHugh, Daley, Carswell, Cummins, Murray, Higginbotham, Hollis.
Provisional Dundee United squad: Cierzniak, Watson, Dillon, Douglas, McLean, Ryan, Millar, Flood, Rankin, Armstrong, Mackay-Steven, Russell, Daly, Gardyne, Gauld, Dow, Souttar, Petrie, Banks.
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