Tom Onslow-Cole has replaced Alan Morrison in the Arena Ford squad for this weekend's round of the British Touring Car Championship at Croft.
The three-times BTCC race winner, who was due to be at the circuit to drive the doctor's car, was only informed of the decision a few hours before Saturday's free practice session.
"It's a shock," he said. "It happened at eight o'clock this morning, but I didn't take much convincing to drop everything and get over here.
"I'm using Alan's race suit and seat, and they both fit pretty well. I'm just thankful for the opportunity, because Arena has got the car pretty well sorted now."
According to the team, Morrison, who scored two points finishes at Oulton Park last time out, is missing Croft due to personal reasons. He is expected to return for the next round at Snetterton in August.