The ex-England international played 66 minutes at the Mercedes-Benz Stadium as Eastern Conference basement boys United lost 3-1 to table-topping Atlanta.
Speaking to his new side's official website after the match, the former Manchester United and Everton ace admitted to having mixed feelings.
"Today was a difficult game. Atlanta are top of the league and they're a good team," he said. "We knew it was going to be a difficult game. We knew it was going to be tough.
"We had to hang in there at times and unfortunately we didn't quite do that well enough today. But there are still a few positives and I'm happy about more minutes on the pitch today which will help me. Overall, I'm disappointed with the result on a personal level."
Rooney also featured for half an hour from the bench against Vancouver Whitecaps last weekend, setting up one of his side's goals in their 3-1 win.