Cooper took a crashing fall off Clarcam, owned by Gigginstown, at the second last in the Fred Winter Juvenile Handicap Hurdle.
He was quickly removed to hospital in Gloucester, accompanied by his father, Tom.
Turf Club Medical Officer Dr Adrian McGoldrick said: “He has been taken to Gloucester and will be going straight to theatre.
“He has suffered a compound fracture of his right leg, he has broken his tibia and fibula. Bryan will be out for between four and six months.”
The timing of the injury is particularly cruel.
Cooper was due to team up with Last Instalment in tomorrow’s Gold Cup, with David Casey aboard First Lieutenant.
It has been confirmed Brian O’Connell will now renew his partnership with the Philip Fenton-trained Last Instalment.
O’Connell was on board the nine-year-old when he landed the Hennessy Gold Cup at Leopardstown in such spectacular fashion on his most recent outing, and is unbeaten in four rides on the horse inside the rails.
O’Connell will also deputise aboard the owners’ Rule The World in this afternoon’s feature, the Ladbrokes World Hurdle.
Rathlin is Gigginstown’s sole representative in today’s Ryanair Chase and David Casey has been called up to take the mount.
Jason Maguire is another jockey in the wars and last night he was brought out of an induced coma after he had part of his liver removed following a fall at Stratford on Monday.
The Grand National-winning jockey was unseated while riding Scots Gaelic and kicked in the stomach by another horse.
The Irishman, 33, was airlifted to University Hospital Coventry, where X-rays revealed he suffered a fractured sternum and bleeding on his liver.
Maguire’s agent, Chris Broad, said the surgeon had been happy with the way Maguire’s operation went.
Broad’s wife, Jane, added: “Jason has been brought round out of his induced coma throughout the day.
“I think he’s very aware of who’s around him — his wife Lauren, as well as Chris — and he’s had a few other very close friends in to see him.
“At the moment, fingers crossed, it’s going in the right direction.”
Meanwhile, Last Instalment will face 13 rivals in tomorrow’s feature with 2013 winner Bobs Worth likely to go off favourite.
Barry Geraghty’s companion banished memories of a subdued return at Haydock in November with a ruthless display in the Lexus Chase at Leopardstown a month later.
Runner-up that day was First Lieutenant, who finished second in last season’s Ryanair Chase and returns to Cheltenham for trainer Mouse Morris and owners Gigginstown House Stud.
Nicky Henderson has a worthy second Gold Cup contender in Triolo D’Alene, who looked the real deal when winning the Hennessy at Newbury on his last outing in November.
Paul Nicholls saddles King George VI Chase winner Silviniaco Conti, who had travelled very smoothly throughout the race last year before he fell three fences from home.
Irish interest is further spiked by the presence of On His Own, who was supplemented for the Gold Cup by trainer Willie Mullins and owner Graham Wylie, Lord Windermere and Lyreen Legend.
Cheltenham specialist The Giant Bolster has finished second and fourth in the respective last two renewals and once again goes to the well for the David Bridgwater team.
Teaforthree, the ante-post favourite for the Crabbie’s Grand National, warms up for Aintree at the top table, while Venetia Williams gives Houblon Des Obeaux and Katenko the chance to shine.
The field is completed by Cloudy Too and Knockara Beau.