And even the stadium announcer was forced to step in as the Red Devils lost 3-1 in America.
An unrecognisable United XI lined up in San Diego, with Erik ten Hag not even present ahead of tonight’s game against Real Madrid in Houston.
A team of kids – and free agent Jonny Evans – were tasked with getting the better of Phil Parkinson’s National League champions.
But United, who had Dan Gore sent off, crashed to a 3-1 defeat under the watch of Under-21 boss Travis Binnion.
And the game was marred by a serious incident involving Wrexham star Mullin and United goalie Nathan Bishop.
The Welsh side’s star man was left flattened and taken to hospital following a barge from Bishop.
Wrexham later revealed Mullin suffered a horrific punctured lung.
And fans of the new League Two outfit made their anger known by booing Bishop throughout the first half.
He was hooked at half-time for fellow keeper Radek Vitek.
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But that didn’t stop Wrexham supporters from jeering the replacement – even though he’d had nothing to do with Mullin’s absence.
The boos got so loud that the stadium announcer was forced to make clear that Bishop had indeed been subbed off at the break, with Vitek innocent of any wrongdoing.
Bishop later tweeted an apology to Mullin, who is set to miss between six and eight weeks with his punctured lung.
The youngster said: “Just wanted to share my sincerest apologies to Paul Mullin. A complete misjudgement and a genuine accident with zero malicious intent at all.
“Wishing you the speediest recovery and hope to see you back scoring goals as soon as possible.”
Mullin also tweeted a selfie from hospital, adding: “Thank you for all your message. All good here.”
However, Bishop’s role in Mullin’s injury left Wrexham boss Parkinson raging.
He said: “I’m fuming about it, I’ve got to be honest with you. It was a clumsy, reckless challenge in a pre-season game and I’m not happy with it at all.
“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.
“It should have been a straight red. If it’s not denying a goal-scoring opportunity it’s still a dangerous challenge, so if you add the two scenarios together he should have been off the pitch.
“It’s a dangerous challenge and obviously Paul Mullin is our talisman. The way the lads responded after that I thought was outstanding.
“That’s what we’re all about as a group – resilience, sticking together.”
United U21 boss Binnion leapt to Bishop’s defence, adding: “It’s part and parcel of the game, nobody wants to see anyone get injured.
“He’s made an honest challenge, he’s come off worse. He’s already reached out to him.
“Booing him, it is what it is. He’s tried to reach out. He’s a great lad Bish, he hasn’t got a bad bone in his body.”
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