Manchester United have no margin for error in the revised Europa League format, Fred has warned.
Ole Gunnar Solskjaer's side beat LASK 2-1 on Thursday (AEST) to wrap up a 7-1 aggregate triumph and reach the quarter-finals.
The impact of the coronavirus on the competition's schedule means ties will be settled in one-off matches, all played in Germany, with United facing Copenhagen on Tuesday.
Should they prevail, the 2016-17 tournament winners would face either Premier League rivals Wolves or La Liga side Sevilla.
"There are two important games, and a single match in the final. You have to make sure you're highly prepared, you can't make mistakes," Fred told United Review.
"In league football, you still have the chance to recover, you make a mistake and lose the game you can still recover later in the competition.
"In tournaments, if you lose a game by a certain margin of goals it's difficult to come back. So you need to concentrate even more. I think that is what’s most important for us. That will be the biggest test."
If United do reach the final on August 21, it would mean playing 15 games in 63 days since their campaign resumed on June 19.
But Fred is not worried about fatigue as he feels the squad are conditioned to such challenges.
"We've got used to these games coming one after another during the intense fixture calendar in June and July," the midfielder said.
"So we're kind of used to it and I think it could be an advantage when playing games that are a few days apart, like they will be in Germany.
"I think it will be an advantage for us in the Europa League tournament format – I'm certain we'll be ready."