The Irish rider underwent back surgery after his mount slipped up on a bumper at the weekend.
Magnier has back surgery after tumble at Down Royal
JP Magnier is expected to remain in the Royal Victoria Hospital in Belfast for a few days after undergoing surgery on his back.
The accomplished amateur rider, son of Coolmore Stud's John Magnier, was involved in a nasty incident at Down Royal on Friday when his mount On The Verge slipped up in a bumper.
Magnier is in a cast after having rods inserted into his back to stabilise his vertebrae.
The Irish Turf Club's chief medical officer Dr Adrian McGoldrick said: "A family friend has told me the operation was very successful.
"He will remain in Belfast for three or four days and then will be discharged home."
Magnier is well known for his association with the Jonjo O'Neill-trained Rhinestone Cowboy, winning two Grade Ones on the high-class gelding, who ran in the colours of his mother, Sue Magnier.