Erling Haaland scored the decisive goal with a far-post strike, which was eventually awarded following a lengthy VAR review.
The result leaves Michael Carrick’s side needing to respond quickly, with United returning to action on Wednesday when Brighton & Hove Albion visit in the Carabao Cup.
Tielemans expressed his disappointment with the outcome while insisting his team must put the defeat behind them and focus on their next challenge.
"It's disappointing, everyone is disappointed and frustrated because we know we can play so much better,” the Begian told MUTV.
"We want to do things well, but today it didn't happen like we wanted.
"We didn't use the right spaces, we made mistakes in our passes and maybe rushed it a bit too much. We didn't make the right last pass in the final third to create enough chances to score.
"The process is try to get our heads together and react in the next games, that's the only thing we can do. We can analyse this game and see what didn't happen, and do better in the next one."