Dragons star striker Mullin was left with a punctured lung and needing oxygen after a brutal coming together with the young United goalkeeper in a pre-season friendly.
Wrexham won the exhibition match against United's youth team 3-1 at the Snapdragon stadium in San Diego, but it was overshadowed by the injury to the League Two side's key player in the 12th minute.
Bishop collided with Mullin outside the area as the pair challenged for a high ball, resulting in the shot-stopper receiving a yellow card and being booed by fans before being substituted at half-time.
The incident lit up social media, with Mullin likely to miss Wrexham's opening matches in their return to the Football League.
United's coach for the game - Travis Binnon - said the 23-year-old shot-stopper was 'heartbroken', while Bishop himself took to Instagram to apologise.
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And keen to make sure that the youngster wasn't too troubled by the incident, Reynolds also reached out according to the Manchester Evening News.
The Deadpool star reportedly wanted to thank Bishop for how he dealt with the incident, which comes in stark contrast to his team's manager, Phil Parkinson.
Caught up in the heat of the moment, Parkinson sent a warning to Bishop, saying: "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.
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"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's a clumsy challenge from the goalkeeper, I'm disappointed with that. It should have been a straight red.
"If it's not denying a goalscoring opportunity it's still a dangerous challenge so if you add the two scenarios together he should have been off the pitch."
Reports from the Northern Echo claim that the act could be Bishop's last in a United shirt, with a move to Sunderland allegedly 'agreed'.