St Johnstone kept in the hunt for a European berth by beating United at Tannadice for the first time in 13 years, thanks to Liam Craig.
McNamara, whose side have lost their last four, said: "I thought we started the game okay, had a couple of chances and then they came into it.
"We were disappointed with the goal we lost, coming from our throw-in.
"I also thought we were a bit predictable at times and didn't ask enough questions of their defence or goalkeeper.
"But we have sustained a lot injuries. Sean Dillon is going in for an operation on Monday and that's a full back-four we have lost since the start of the season.
"We had a 16-year-old (John Souttar) and a 19-year-old (Luke Johnston) making his home debut, the positives from that are they are getting experience. The negative is it is a difficult time to come in with the way results are going.
"But we have to dust ourselves down and battle on for next week.
"The effort is there but obviously St Johnstone are playing for a Europa League spot, they have had a good season, they work hard and defend well.
"I just think we were a bit predictable and put ourselves in trouble with square passes and we didn't cause them enough problems in the final third and when you do that, you put so much pressure on your defence."