The striker is set to miss the start of the League Two season after Mullin was left with a punctured lung following a collision with United keeper Nathan Bishop.
The Red Devils took on Wrexham at the Snapdragon Stadium in San Diego where Erik ten Hag fielded a team mainly full of youngsters.
Early in the contest, Bishop brought Mullin down and he needed lengthy medical treatment on the pitch before it was later confirmed that he had suffered the sickening injury.
Hollywood star Reynolds took to Instagram to bemoan the loss of his top striker, saying: "Massive thanks to @manchesterunited for the incredible opportunity to play against a legendary Premier league side.
"But Jesus…. not worth losing Paul Mullin. F'n Brutal."
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In a separate post, the Deadpool actor jokingly revealed that fellow Wrexham co-owner Rob McElhenney was having stern words with Bishop.
Talking to UK Prime Minister Rishi Sunak, Reynolds said: "With Deadpool paused, we visited 10 Downing to discuss the excellence of UK film crews and @maximumeffort.
"Aside from shooting back home, the UK is my favourite place to make a movie.
"Hoping to see even more film work head to Wales.
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"Rob would have joined but he's currently having a word with Man Utd's youth team goalie."
Mullin needed oxygen on the pitch after getting clattered by Bishop in a match that also saw United starlet Daniel Gore sent off.
Wrexham emerged 3-1 winners to claim a famous Premier League scalp despite the ill-tempered nature of the game.
And the Dragons striker will now have to stay in the United States as he receives treatment on his lung, which could rule him out for up to two months.
Wrexham boss Phil Parkinson 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 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."