"Everything; everything," he told BBC Radio Scotland. "The commitment; the concentration; desire. Sometimes you just get the feeling that the end of the season's there.
"A lot of things were frustrating today. At times we looked a bit predictable and slow in possession."
United, who remain sixth in the Scottish Premier League, had started positively and nearly opened the scoring through midfielder Stuart Armstrong.
However, Saints' grew into the game and, after Liam Craig had put them in front, only a good save from goalkeeper Radoslaw Cierzniak prevented the Perth side doubling their lead near the end.
It is the first time St Johnstone have beaten United at Tannadice since August 2000.
"Yes, it bothers you," McNamara said.
"We caused our own problems as well at times - their goal comes from our throw-in.
"Our flair players, like GMS [Gary Mackay-Steven], we want to get him on the ball and get him one on one, but we didn't really have a lot of creativity with that side of things today. That's the frustrating part."
United's 19-year-old defender Luke Johnston made his home debut but was withdrawn in the second half for striker Johnny Russell, who made his come-back after sustaining a broken leg in March.
"I'm not being too hard on the defence side of things and I feel sorry for the goalkeeper," added McNamara.
"Three lads were out today and Sean Dillon's going in for an operation, so that's the full back four out that started the season.
"So it's frustrating for me not having the strongest side available, but at the same time it lets me see what I've got and what I need for next season."
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