The Red Devils struggled for much of the contest at the Brentford Community Stadium, but they looked to have stolen all three points at the death, with Mason Mount netting his first goal for the club in the 96th minute.
Brentford refused to be beaten, though, and they levelled the scores at 1-1 in the ninth minute of added time at the end of the 90 through central defender Kristoffer Ajer.
Ten Hag has admitted that he was "not happy" with his side's display in the capital, claiming that the Red Devils lacked "aggressiveness" in their first game after the March international break.
"Don't lower the performances from Brentford because I have seen them against Man City and City were also lucky to win that game," he told reporters after the clash.
Ten Hag unhappy with team's display in London
"I have seen them against Liverpool and in the first 35 minutes Brentford should have taken the lead. We should have done this better. I'm not happy with the performance, it's no misunderstanding.
"The players are also disappointed by the performance but we should be consistent in the way we take a game, we [need to] go in a game and perform and show more aggressiveness especially.
"We made some mistakes and we have to talk about this of course. We should act differently in that occasion where we conceded the equaliser, we should have acted differently in that situation.
"As I say if you are in stoppage time and you score that goal, which maybe we didn't deserve, then I have to make compliments to the team that they hang [on] in the game. They were fighting but I did think Brentford wanted it more during the whole course of the game but we have to take it over the whole game, take it [beyond] the time.
Ten Hag delighted with Andre Onana's performance
"You have to defend the box and that was what we did quite well. We had a great goalkeeper, he was brilliant tonight, we have to say that. I think our results are quite consistent from January on.
"We concede shots but we don't concede so many goals. We're creating chances, with Rasmus Hojlund big chances. We're scoring a goal and it's all about bringing that over the line.
"I thought we were good in certain situations and when we were winning, we didn't deserve it, but we should take the winning and do so more often. We were fighting and we created some great chances and Andre Onana was fantastic tonight.
"Rasmus Hojlund had a great chance. When you make the winning goal that is when you must bring it over the line. I am very pleased for Mason [Mount] but I also feel sorry for him as it should have been the winner."
Man United will be aiming to return to winning ways in the Premier League when they head to Stamford Bridge to tackle Chelsea on April 4.