The 32-year-old Peacock has been told he needs a knee re-construction following the injury he sustained in Friday's 38-6 Engage Super League win at Castleford which will sideline him for six to eight months.
It means the Rhinos will be without their inspirational forward for their first Carnegie Challenge Cup final for 11 years and leaves England looking for a new captain for the end-of-season Four Nations Series in Australia and New Zealand.
"I am stunned to be out of action for six to eight months and it has been a bit bewildering if I am honest," said Peacock.
"The injury did not feel any worse than previous knocks but the scans have revealed the worst possible news."
Warrington captain Adrian Morley, who was England vice-captain under previous head coach Tony Smith, will be a likely candidate to take charge of the national team for the clashes with New Zealand, Australia and Papua New Guinea in October and November.
More immediately, the injury is a body blow to Leeds as they bid to hang onto their Super League crown and end their 11-year wait for a Cup final trophy.
The Rhinos had already ruled out of their Wembley plans one front rower in Luke Burgess after he suffered a double fracture to his jaw bone in Friday's match and they may now turn to youngster Luke Ambler, who made one of only two starts for the club in the fifth-round win over Blackpool in May.
Peacock, who twice won the Cup with Bradford, was desperate to play for his home-town team in the final and to make a first appearance at Wembley against Warrington on August 28.
"Personally it is obviously very disappointing news," he added. "It just goes to show how cruel a game rugby league can be and I now have to do everything possible to get myself back fit for the start of next year.
"I won't be having the operation until after Wembley so I will be able travel with the team to be part of their preparation and help them out in any way I can.
"I am sure the lads will rise to the challenge. We have a great squad with great players and I know they can do themselves proud at Wembley."
Leeds head coach Brian McClennan said the loss of Burgess and the talismanic Peacock would provide the rest of their team-mates with additional motivation for their crucial run-in.
"We are devastated for JP," he said. "He has worked so hard this year to get us in the current position we are in with a chance of fourth, in a good position for the play-offs and through to the Carnegie Challenge Cup final.
"We have now lost two of our props and that gives us an extra challenge to lift up and go forward.
"JP will be around the camp and doing everything he can to help the team. What it comes down to now is every player fortunate enough to take to the pitch for Leeds to lift us up.
"We always say before every final that the 17 who go out on the field are not just doing it for themselves but that they represent their team-mates and our great club.
"Everyone puts the Rhinos first, that is what we do, and I know that they will make both JP and Luke Burgess proud of their efforts."