A couple of late goals and a couple of late red cards condemned Minnesota to a third league defeat of the season.
United were up 3-2 on Toronto FC with just 13 minutes to go when Toronto struck twice in the space of three minutes, with a double from Jordan Hamilton giving the home team a 4-3 lead.
Any hope of a Loons comeback were shot when Jan Gregus and captain Francisco Calvo were sent off in the 89th and 91st minutes.
Earlier, a double from Darwin Quintero – one a penalty – and a strike from Angelo Rodriguez had given United the advantage.
Head coach Adrian Heath said that his team lost against "one of the best teams in the league."
"I thought we looked very good going forward, we had a good shape about us, but we’ve conceded poor goals again," he told MNUFC.com. "It’s another game where we’ve scored three goals and come away with nothing. It’s something we have to address.”