Having fallen three goals behind to a dominant United side, Motherwell were awoken from their slumber by a penalty conversion from John Sutton shortly before an hour, though they rarely looked like building on that goal.
The Tannadice side, meanwhile, face Hibernian at Easter Road next Friday night and McNamara admitted the performances of Gaving Gunning and Ryan Dow, their goalscorers last night, gave him plenty to think about in terms of his team selection for that encounter.
"Our play in the first half was as good as it gets," he said, "but I thought the second half was a bit stop-start; it was horrible. The penalty gave them a lift but in the first half we were excellent. We moved the ball well and showed good energy and had many chances to put the game to bed, really."
Gunning, playing his first game for five weeks, replaced the injured John Souttar and ended the night with two goals to his name, while Dow, who opened the scoring following a defensive blunder by Motherwell's Shaun Hutchinson, came in for Ryan Gauld, another on the treatment table.
Stuart McCall, the Motherwell manager, may have bemoaned the unavailability of a clutch of players through injury, while midfielders Keith Lasley and Stuart Carswell played despite being less than fully fit.
Henri Anier, another suffering from the stresses and strains of SPFL Premiership football took his place on the bench.
Meanwhile, former Morton and Queen of the South player Craig Reid, signed as a free agent yesterday, was press-ganged into the role of substitute alongside names largely unrecognisable outside Fir Park or, indeed, their own homes.
Reid, though, who made his debut in the second half replacing Lionel Ainsworth, was no stranger to his team-mates and had earlier told the Motherwell website: "I came in during the summer and I am familiar with the place, the boys and the management here.
"I was disappointed things didn't work out in the summer as I really wanted to come here, but Stuart McCall was always honest with me and I understood and appreciated that. I am here now, though, I feel fit and I am looking forward to getting to started."
Despite his selection difficulties, McCall was in no mood to make excuses about the manner of the concession which left them with a mountain to climb.
"Sloppy defending cost us the first two goals," he said. "We could have crumbled but we didn't. It's our second defeat in 10 games and the character we've shown is still there."
It was clear from the early exchanges that both sides were showing an appetite to produce the passing football of which they are capable and John Rankin, United's defensive midfielder, fired in a useful shot from distance which shaved the left-hand post of Gunnar Nielsen, the Motherwell goalkeeper.
Then Dow's clever flick into the Motherwell area found Andrew Robertson, named this week in the Scotland squad for the forthcoming friendly in Poland, before Shaun Hutchinson intervened.
United's pace and quick-thinking left their opponents chasing shadows and when Hutchinson's pass back to his goalkeeper from 35 yards fell woefully short in the 22nd minute, Dow pounced on the chance, racing in on goal and stroked his shot into the side of the net.
Opportunity knocked for the Fir Park side in the 35th minute as Sutton's forward flick found Iain Vigurs two yards from goal and the former Ross County player would have scored but for the acrobatic intervention of Radoslaw Ciernzniak, the Tannadice side's goalkeeper.
McNamara's men continued to play attractive football and when Dow's corner from the left four minutes before the interval was headed on by Nadir Ciftci, Gunning was waiting at the back post to touch the ball home.
It was richly deserved on the balance of play - although United did survive a scare a minute later when Gunning appeared to go through Vigurs inside the box. However, referee Bobby Madden did not award a penalty.
The Irish defender's good evening got even better immediately after the break when Madden awarded a free-kick for a Vigurs foul on Ciftci 35 yards from goal. Gunning's strike, albeit aided by a fortuitous deflection, proved extremely effective as it beat the Motherwell wall and Nielsen to seal the points for his side and continue their excellent run.
Motherwell were offered a ray of hope soon after, as John Sutton struck from the penalty spot following Cifti's needless handball in the area. And, despite the predictable lift it gave the visitors, the goal never looked like upsetting United or altering the outcome of the match.
Indeed, United had the chance to reinforce their lead on the hour mark but Stuart Armstrong could not keep his shot down from 12 yards.
Gary Mackay-Steven also had a chance against the run of play with 10 minutes remaining, only for Nielsen to foil his low shot from 10 yards.
But the hosts held out with plenty to spare to move to within seven points of second-placed Aberdeen.