The Terrors lost 3-0 to St Johnstone in Perth as Stevie May grabbed a hat-trick while the visitors finished with 10 men for the second match running.
But the main talking point came half an hour before kick-off when they team sheets revealed the Tannadice manager had made a string of changes to his side following on from their 4-1 Boxing Day defeat to St Mirren.
And McNamara insisted there were further injury concerns he had to contend with.
He said: "Injuries were behind it. A few of the boys were carrying knocks but I took a gamble by putting (Gary) Mackay-Steven and (Stuart) Armstrong on the bench after they passed a fitness test this morning.
"(Ryan) Gauld is out with a groin problem, Gavin Gunning is struggling with his knee and Keith Watson is obviously suspended after being sent off in the last game. So a lot of it was forced.
"The only one that wasn't forced was leaving out John Rankin for (Morgaro) Gomis. John has played a lot of games
"Putting Mackay-Steven and Armstrong on the bench was a gamble and hopefully they'll be okay for Wednesday. I don't know how many of the boys will be fit for the Aberdeen game, hopefully I'll have a better idea tomorrow."
With a patch-work side as opposition, Saints made the most of the opportunity and claimed their first home win over their Tayside rivals since November 2000.
Neither May nor any other Perth player had found the net in their previous four Scottish Premiership games but May ended his 460-minute wait when he stuck away a penalty midway through the first half after United defender Calum Butcher had caught him with a clothesline in the box.
Terrors skipper Sean Dillon had undercooked a headed back pass and Butcher's last-man foul cost him a straight red, while May made no mistake as he rolled the spot-kick straight down the middle.
McNamara added: "The sending off killed us with that happening 15 minutes into the game and obviously them getting the opening goal from the penalty.
"It was the turning point of the game and I'm disappointed with the concentration at that point. I said to the players before the game that the main threat from St Johnstone would be balls from the corners or over the top, so we had to be switched on."
May's second on 57 minutes saw him rifle a powerful strike into the corner of the net from an improbable angle before he poked home from Gwion Edwards' cross two minutes from time to complete his first top-flight treble.
Boss Tommy Wright was delighted with his striker's display but claimed his side would have seen off United regardless of referee Stephen Finnie's red card call.
"It's never easy against Dundee United but I thought we started the game really well," he said. "We totally dominated and were by far the better team.
"I just hope the performance is not over looked by the fact they had a man sent off because even with 11 men, I don't think they would have been able to compete with us today, because I thought we were excellent.
"Stevie has had a good season and it was maybe a wee test for him today. He hadn't scored for a few weeks but he works hard and is always a threat. I'm really pleased that he got his hat-trick.
"Hopefully he can kick on again and go on a wee run again.
"I'm pleased with Stevie but just as I am with the rest of the lads. Stevie will get the headlines but it was an excellent team performance."