Smith has been driving the #78 Furniture Row Racing car this season, but was recently told that he had lost the ride for 2013 and would be replaced by Kurt Busch starting at this weekend's race in Charlotte.
It had previously been announced that Smith would effectively swap seats with Busch and take over the driving duties of the #51 Phoenix Racing car, but the news of Earnhardt's concussion has changed those plans and left James Finch's small team needing an experienced Cup driver at very short notice.
They've plugged the gap by turning to AJ Allmendinger, the driver who was suspended from NASCAR competition over the summer after failing a random drugs test, but who was confirmed last month as being eligible to drive once again after successfully completing the series' "Road to Recovery" rehabilitation program.
Allmendinger was first from his previous team, Penske Racing, after the drugs test result was confirmed. Coincidentally, he had been in the team's #22 car previously driven by Kurt Busch before he and the team fell out at the end of the 2011 season, making this the second time that Allmendinger will have followed Busch into the same race seat in under 12 months.
There had been speculation that Busch himself might not be able to compete this weekend at Kansas, after a bizarre incident early in the race at Talledega where he drove away from track safety officials with an EMT kit still on the top of his car, apparently ignoring instructions from race control.
However, NASCAR were satisfied that being excluded from the race was sufficient penalty and accepted his explanation that he had removed his helmet when talking to the track workers and was unable to hear orders from the tower or his pit wall, and that therefore he would not be given an additional race ban penalty.