Jonny Evans feels United aren't safely through in the Europa League yet despite beating Ajax 2-0 on their own patch.
The centre-back admits the Barclays Premier League champions did not hit top form in Amsterdam and is wary that the Dutch side could pose problems at Old Trafford next week.
"We probably didn't play as well as what we'd have liked and controlled the game but it was Ajax's game," he told MUTV. "They were the home team and play a similar way to Barcelona. They are very open and like to play it out from the back.
"There were some harsh words from the manager at half time as we didn't get to grips with things in the first half. I think we were a bit too open ourselves but we got a better shape and went from there.
"I don't think anything is safe in Europe, especially the Champions League. Even in our home games, we probably didn't manage to get the right results, so we'll be looking to keep it tight at the back at Old Trafford and maybe stamp our authority on it a bit more."
While dropping into the Continent's second competition was a new experience for United, Evans insists there was still plenty of allure in a contest with the Dutch champions.
"No matter what, a game in the Europa League is still a European tie and they're still special nights," he added. "I'm sure the fans enjoyed it."