The shock penalty shoot-out loss to Villarreal sticks in the craw heading into the new season, when they will look to challenge for their first league title since 2013 and major silverware since 2017.

"We need that (toughness) to be able to challenge at the top," United boss Solskjaer said. "We can't be too nice.

"We can't have, 'oh, there's another game next week' – every single game and every single training, it matters.

"If you want to be successful, that's the discipline, that's what you need and hopefully – and it looks like it – they've had a horrible feeling in their stomach as I've had this summer.

"The last game ending the way it did with one penalty hurts and it takes time to get over it, at least until the next game. It's two and a half months ago and it's horrible.

"If we want to win, we've all got to have that discipline and that hunger and that fear of losing.

"You have to have that edge of being afraid of that failure – fear of failure, it's a good feeling to have."

Solskjaer said "nothing is beyond these players" and began plotting for the new season immediately after losing to Villarreal in Gdansk "to make sure it doesn't happen again".

"It's a horrible feeling and that feeling came back when I watched the penalty shoot-out with Chelsea v Villareal the other day (in the Super Cup)," Solskjaer said.

"It's like we were there and every time you see those pictures it's not nice.

"Every time I see Arsenal celebrating the Premier League from 1998 or 2000 or whatever years that we didn't win, it's horrible.

"These are the moments that you go back into the heart and it doesn't feel good.

"You play football to be successful, to win, and those are the moments you want to win."

United kick off the new campaign against rivals Leeds on Saturday lunchtime, when £73million signing Jadon Sancho is in line to make his debut at a packed Old Trafford.

The winger, who featured for England at Euro 2020, only reported to training at the start of the week, while Edinson Cavani is in isolation after an extended pre-season break.

"It's not about giving them extra time off," Solskjaer said. "It's about giving them the time off they need to recover.

"Footballers nowadays play 60/70 games and at a higher intensity level, more scrutiny, social media. They need to switch off.

"We have to look after players. We can't just add games and add games and add games and say if you're successful that means you have less time.

"I always go for a minimum of three weeks you need as a break to mentally switch off, physically recharge.

"Some have had four weeks, some have had five for different reasons, some personal reasons.

"They've had a shorter pre-season now, say Fred, Luke (Shaw), Harry (Maguire), Jadon because they went so far in the tournaments.

"Edinson – that was a different matter, that's some personal reasons.

"I wouldn't say that we give them extra time off. We need a product, we need players to play well, we need them to be fresh.

"The world hasn't been perfect so it's quarantine and isolation and it's a difficult situation for them but I'm sure we'll reap the benefit when they've really got going.

"Of course some of these players have only played 45 minutes, some have played a couple of games and normally you'd need four or five games after a long holiday.

"But say Fred with three weeks, that's not a long holiday, so they're all ready."