An optimistic Old Trafford crowd fell flat for much of the first game of the English top-flight season after watching their team struggle to create openings.
When they did present themselves, they were mostly squandered, until substitute Joshua Zirkzee netted a late winner on his debut.
"I think one area we have to get right is how to kill (games) in the box," Ten Hag told reporters. "We saw it in the friendlies and last week against (Manchester) City (in United's Community Shield loss).
"We created enough chances and should not to have wait for a late winner. We have to be more composed and kill teams.
"Maybe it is because it is the start of the season, players have to come into full speed. We have enough players with scoring abilities."
That late winner came via the boot of Dutch forward Zirkzee, stepping off the bench to convert a late Alejandro Garnacho cross.
"He (Zirkzee) had an injury at the end of his career at Bologna," said Ten Hag. "We have to build up some deficits.
"He has some attributes we didn't have and straight away he showed it. He is very good in his linking up combinations. We have some very good ball-playing combinations in the team, but he has to arrive in the box to score goals and he did that tonight.
"He is a Man United player and it is very good for a striker coming in and scoring his first goal."
