Boss Phil Parkinson subsequently warned Bishop to "steer clear" of Wrexham following the "reckless" challenge.

Goals from Elliot Lee, Aaron Hayden and Sam Dalby sealed Wrexham's victory.

Spanish 19-year-old Marc Jurado scored in first-half injury time for United, who had Dan Gore, 18, sent-off shortly after the restart.

Ryan Reynolds was unable to attend but Wrexham co-owner Rob McElhenney was present to see his side earned a memorable win in front of a sold-out 34,248 crowd at Snapdragon Stadium in California.

United boss Erik ten Hag, whose senior players have travelled on to Texas before tomorrow's match against Real Madrid, watched his youthful side's performance from the bench.

The Premier League club's starting line-up also featured the experienced Jonny Evans, who has returned on a short-term deal after leaving Leicester City at the end of last season.

Bishop apologises but is warned to 'steer clear' of Wrexham

Mullin scored 46 goals in all competitions as Wrexham ended a 15-year absence from the football league last season.

But the striker will now miss the start of the campaign following his collision with 23-year-old Bishop, who received a yellow card for the foul.

Mullin required lengthy treatment before walking off the field with an oxygen mask around his neck early in the first half.

Manager Parkinson expressed his anger after the match: "It is a real blow for us, you can't hide away from that. I thought it was a clumsy challenge from the goalkeeper and it should have been a straight red."

He added: "It was a dangerous challenge and Mulls is our talisman. I'm fuming with it, I have to be honest about that. It was a clumsy, reckless challenge in a pre-season game.

"I haven't seen the goalie and he's probably best steering clear of us for the time being because we're not very happy."

Bishop sent an apology to Mullin on Twitter: "Just wanted to share my sincerest apologies to Paul Mullin. A complete misjudgement and a genuine accident with no malicious intent at all!

"Wishing you the speediest recovery and hope to see you back scoring goals as soon as possible!"

Lee opened the scoring after 29 minutes and Hayden headed in a second seven minutes later - but Jurado's side-footed finish halved the deficit.

United's Gore was dismissed after receiving a straight red for a tackle on Andy Cannon early in the second half, before Dalby headed in Wrexham's third from close range.

Wrexham owner Rob McElhenney was in attendance to see his side's victory

Elliott Lee opened the scoring for Wrexham, who lost 5-0 to Chelsea last week

Marc Jurado spent time at Barcelona's academy

Dan Gore signed a new contract at Manchester United on Monday

Wrexham goalkeeper Ben Foster played for Manchester United from 2005-2010