Harry Maguire believes Manchester United's inconsistency has been one of their biggest problems this season.
When the season was suspended due to the coronavirus pandemic in March, United were fifth in the Premier League but still alive in the Europa League and FA Cup.
Ole Gunnar Solskjaer's men beat Manchester City and Chelsea twice in the league, as well as Tottenham, but dropped points against the likes of Aston Villa, Watford, Bournemouth and West Ham.
United captain Maguire said his team had been their own worst enemies during the campaign.
"As a Red, our toughest opponent, probably, is ourselves. I would say that at times this year, as a team, we have probably let ourselves down by being a bit too inconsistent, not just for a full game but during games," he said in a Q&A with fans, published by the club's website.
"There have been times when we've been so good in games and then we've come off it a bit and been punished. So I'd say our toughest opponent is ourselves. We need to be mentally strong.
"We are a young group but we don't really want to use that as an excuse. We want to be young but play mature. That is where we have really picked up in recent weeks."
The Premier League is reportedly looking to restart its season in June as it tries to finish the campaign behind closed doors.
Maguire hopes his team-mates return to training in good condition ahead of the campaign resuming.
"Well, I think all that you can do is look after your own fitness. Obviously, we speak in group chats and I'm sure all of the players are doing that," he said.
"I think you have got to get a base fitness so that when we start back training, we are at a level of fitness where we can go straight into high-intensity training with footballs.
"To get match fit, you need to be training with the lads and playing competitive games. We need to look after our own base fitness, do our gym work, keep our muscles strong, and I feel like we'll be fine.
"As a group of lads, we are all so dedicated and hopefully we can start the season again like we ended it."