Afterwards Ten Hag questioned whether his squad are good enough to string a run of wins together after failing to build on the result in midweek.
"I think, as a squad, we are not good enough to be consistent and we have to work as a squad to improve that," said Ten Hag.
"As a group, we have to improve, we have to get tougher, be ready for the game, from the start. I said it can't be in this league if you are not playing on the highest levels, because you get killed."
United, who have now lost 11 of their 23 games this season, face a crucial week with Bayern Munich visiting Old Trafford in the Champions League on Tuesday before a trip to Anfield to face Premier League leaders Liverpool on Sunday.
Bournemouth at home should have offered Ten Hag's team the chance to build some momentum ahead of two massive games but instead they slipped to their sixth defeat of the season at Old Trafford.
"Of course, I am annoyed, disappointed, definitely," said Ten Hag.
"I expected something different. I hoped before the game you can build on but the performance and result from Chelsea, so it's very disappointing.
"After the 2-0, we broke down so we have to do things better.
"The way we started is no good, that is poor, it's first five minutes of the game and especially against an opponent like today, very good transition team, you give them perfect circumstances, it shouldn't happen but we did everything to turn in the game, we had a lot of attacks, we didn't come to a point to turn the game although we had opportunities.
"I have to prepare the team so they are ready for the game. From my point of view I'm very disappointed with the way we started and we have to be better.
"We are really inconsistent, we have the abilities to do it, but you have to do it every game and every third day."
Afterwards, Ten Hag also addressed the fan reaction to his decision to substitute Anthony Martial in the second half. Martial was picked to start ahead of Rasmus Højlund but was replaced after 60 minutes to loud cheers from the United fans.
"Two weeks ago against Everton, he played a very good game," said Ten Hag.
"He absolutely has the competences and abilities to play, so I don't think it's fair after a performance like this to go to one player, we as a group failed altogether."