The Manchester United youngster inadvertently punctured the lung of the Wrexham striker during a pre-season clash on Wednesday night.
This led Mullin to hobble off the pitch with an oxygen mask in place, with supporters booing Bishop for his role in the incident.
The clash occurred when Mullin beat Bishop to a lofted through ball on the edge of the area in the 12th minute before being whacked into the chest.
Foster knows exactly what it's like to be a young goalkeeper for the Premier League giants having joined the club at the age of 22 and took the time to console Bishop after the high-profile incident, making his way into the United dressing room to see the young stopper.
"I feel so bad for Nathan, he's absolutely heartbroken," he said.
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"I passed Paul's number onto Nathan so he can get in touch with him but, yeah, he's absolutely gutted. I feel for the lad."
The 23-year-old goalkeeper's regret was clear to see as he took to social media after the match, saying: "Just wanted to share my sincerest apologies to @PMullin7.
"A complete misjudgement and a genuine accident with malicious intent at all!
"Wishing you the speediest recovery and hope to see you back scoring goals as soon as possible!"