The incident occurred in the first half of a youthful United team's 3-1 defeat to the Welsh side in San Diego, before the Reds lost Dan Gore to a harsh red card after the break.
It came as the centre-forward chased a long ball over the top and was a shuddering coming together between the pair, with the United man also appearing to take a hefty blow.
Mullin was later taken to hospital to have the problem assessed. Thankfully, the forward has since uploaded a photo on social media, reassuring supporters that he is in decent shape: "Thanks for all your messages.