The champion jumps jockey twisted his hip in a fall from Springtown Lake, who looked all over the winner when coming down after jumping the final flight in a handicap hurdle over two and a half miles at the Somerset venue.
Johnson gave up his rides at Ascot on Friday, but intends to return at Haydock, where he has four mounts.
"I twisted my hip so I'm not riding today, fingers crossed I'll be back tomorrow," he said.
"I just landed awkwardly when he fell a couple of strides after the last.
"It's a bit sore, but I'm going swimming and hopefully all will be right for tomorrow."