Dons boss Jimmy Calderwood, who has seen his team only pick up one point at home after four matches, was again frustrated with the performance.
He said: "That's six goals we have conceded this season and probably all of them have been the result of individual mistakes.
"I think Tommy got shoved but his hands should never be in that position or you are going to get decisions like this against you. It was a penalty and the boy has finished it well.
"Last week against Hamilton, it was two individual mistakes that cost us, and on the first day of the season, it was Jamie Smith who got caught in possession that cost us.
"We have to cut that out because we are not giving ourselves a chance.
"Dundee United took a lot of confidence from that and it suits them to play go and play on the counter-attack while we have to go and create chances."
However, Calderwood was pleased with the commitment his players showed.
The Dons boss added: "I can't ask for any more from the boys in terms of effort. They have given everything they have got and kept pushing forward until the end and that's all you can ask for."