Manchester United must avoid conceding the opening goal in their Champions League round-of-16 second leg against Atletico Madrid on Tuesday, according to Ralf Rangnick.

The Red Devils set themselves up for failure in the first encounter by allowing Atletico Madrid attacker Joao Felix to score after only seven minutes, but a timid United were able to salvage a 1-1 draw thanks to a late Anthony Elanga equaliser.

Rangnick has reminded his players, though, that they may not be as fortunate this time, with a quarter-final spot on the line for two teams facing terrible domestic campaigns.

"It's critical [not to concede first], even more so against a team like Atletico, who don't mind having the ball," the interim manager stated during his pre-match press conference.

Man United fell behind 1-0 in the first leg thanks to Felix's header.

"I mean, in their last four or five games, including league games, they have had less than 50% possession of the ball.

"We are aware of that and need to ensure that we do not concede at all, but it would be critical to score the opening goal ourselves."

Rangnick has admitted, however, that the Red Devils will require assistance in order to defeat Diego Simeone's side, who won La Liga last season but are currently fighting for a top-four finish this season.

Manchester United's famed German coach wants the Old Trafford crowd to be in "top form" - much like they were in Saturday's dramatic 3-2 victory over Tottenham Hotspur, in which Cristiano Ronaldo scored an incredible hat-trick.

"Yes, I believe that, similar to the game against Tottenham, we will require both our players and our supporters to be at their best on Wednesday night," Rangnick pleaded.

In the end, our supporters can not only be our 12th but also our 13th player on the field and make the difference, making it a very special experience for everyone." "Our supporters play a vital role, and it's always a very special atmosphere at Old Trafford, even more so in international games."

Yet, at the end of the day, it's clearly our responsibility, and we must create those moments, with the rest, in my opinion, being done automatically by our wonderful supporters.

United have been given a boost by the return to action of star midfielder Bruno Fernandes, who missed the win over Tottenham due to a positive test for Covid-19. However, there is still one question mark hanging over Rangnick and his teammates.

"As things stand right now, we have all of our players available for the game tomorrow, with the exception of Luke Shaw, who is a small question mark," the 63-year-old revealed.

In the meantime, all of the other players are back in training and ready to go for tomorrow. "He has been training yesterday and today, but we will have to wait and see if he is fully fit to be available for the game tomorrow."