Adrian Maguire is still being troubled by a torn ligament in his back but hopes to return to race-riding next week.
Maguire has not ridden since Musselburgh on Tuesday last week having suffered back spasms at the Scottish track but had thought he might be back this weekend.
He said today: "It is coming along slowly but it is not right yet and if I come back too soon I could end up back at square one - I just can't rush it but I would like to think I will be back early next week.
"I am having physiotherapy every day and I am using the exercise bike but it is frustrating and I have missed enough winners already!"
Maguire is disappointed at having to miss the ride on Ballinclay King at Ayr on Saturday and he added: "Ferdy (Murphy) has some other nice runners up there including Ackzo and I hope they all go well."
Adie Smith will replace Maguire on Murphy's runners at the Scottish course.
Murphy said: "It is a pity Adrian won't be able to ride on Saturday but I have booked Adie Smith for my runners at Ayr.
"Ballinclay King is in good form and we are sending him over two and a half miles so we will see what options we will hopefully have for Cheltenham."
