Mullin was offered the chance to stay at Wrexham co-owner McElhenney's LA mansion to recover from a punctured lung.
The star striker suffered the gruesome injury in a pre-season friendly against Manchester United last week in San Diego.
After knocking the ball past goalkeeper Nathan Bishop, Mullin was taken out and needed medical attention on the pitch, including oxygen.
He was eventually brought off and was diagnosed with a punctured lung following the hefty blow and is set to miss the start of the season.
Doctors told him he cannot fly home until he has recovered, which could take up to two months.
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It means Mullin remains in the United States and McElhenney informed his main man that he could stay at his as he returned to fitness.
But Ker has since revealed that the 28-year-old rejected his Hollywood boss and is hitched up in an apartment with pals instead.
"I think Rob said, 'he's going to stay with us' and people took that as he was literally going to stay in Rob and Kaitlin [Olson's] house," Ker told the RobRyanRed podcast.
"That offer was on the table but Paul, understandably, wanted to use this time to have a bit of a... here's the thing, I wouldn't want to stay in my boss's house for ten days to two weeks.
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"I think he was very grateful and no doubt will see plenty of Rob while he is in LA but he just wanted a bit of space to himself where he could concentrate on his recovery and R&R (rest and relaxation).
"He's here, he's in LA and he's got some friends out here from home who have come out to stop him from being lonely and to keep an eye on him and help him out.
"Hopefully by the end of this week or start of next week he'll be able to come home."
Wrexham face MK Dons on Saturday in their first Football League match in 15 years following their promotion from the National League last season.
But prolific forward Mullin not being available is a huge blow to the Dragons. Last season, the Liverpool-born star scored 38 goals in the league to fire Wrexham to promotion while he has 64 goals in 84 total league appearances for the club.