Erik ten Hag said he is "convinced" his Manchester United squad will have been strengthened by the end of the transfer window.
United have been heavily criticised by supporters after only bringing in Lisandro Martinez, Christian Eriksen and Tyrell Malacia this summer while so far failing to complete a deal for top target Frenkie de Jong.
Ten Hag is still hopeful of landing De Jong while the club remain in talks to sign Adrien Rabiot from Juventus, and the United boss said it is imperative improvements are made before the transfer deadline on Sept. 1.
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Asked at a news conference on Friday whether he believed he would have a better squad at the end of the window, Ten Hag replied: "We have to. I am convinced we will have.
"I think we're co-operating really well [with the club]. I cannot talk about players under contract at other clubs."
The result has only served to darken the mood around Old Trafford but Ten Hag said he has learned lessons from the game against Graham Potter's side.
"I analyse the game, what went well, what went wrong, where can we improve, what do we have to improve, how we can train and we tell the players and show them how to improve to get solutions," he added.
"We learned a lot but I think it's normal when you start the season.
"It's a new start and you make mistakes. Football is a game of mistakes. We know we have to improve in many things -- it is clear."
United will again be without Anthony Martial against Brentford as he continues his recovery from a hamstring injury. Cristiano Ronaldo, who came off the bench against Brighton, is available but Ten Hag refused to be drawn on whether the 37-year-old could start at the Brentford Community Stadium.
"He had a good training week," Ten Hag said.
"I thought he played a little bit longer than half an hour, he's had two half games and for the starting XI we will see tomorrow [Saturday]. My decision, I keep it [private]."