Erik ten Hag stressed Manchester United must not let their grip slip on games after their EFL Cup semi-final first-leg win over Nottingham Forest.
Goals from Marcus Rashford, Wout Weghorst and Bruno Fernandes delivered the Dutchman a convincing 3-0 win at the City Ground.
But Sam Surridge saw a would-be equaliser chalked off in the first half, in what proved to be a lucky escape for the visitors.
Ten Hag was delighted to see his side bounce back to winning ways after a Premier League loss to Arsenal but cautioned his side to watch their control in games.
"There was one moment that could have changed the game," he told Sky Sports. "That is where we need to improve.
"They changed the system, and we didn't anticipate that. It cannot happen. This is still where we have to learn and improve if [we] want to be top. These moments can't happen.
"We almost let them come [into] the game. We can't allow an opponent to get back when we started so good. We need to avoid that.
"I think in the second half, we were much more composed, and we tracked them. It was much better."
Striker Weghorst, who netted his first goal following his switch from Burnley, echoed his manager's sentiments.
"We didn't have the control like we wanted and [it] made it too up and down after," he added. "We were lucky their goal was disallowed.
"We were not happy with the way we had played in the first half [but] the [goal] at the end it made it even better. It's a great result for us tonight, [so] let's finish it next week."
United will host Forest at Old Trafford next Wednesday for the second leg.