The skipper will miss both legs of the semis after picking up a calf injury during training and is only rated "touch and go" should Leeds reach the final at the Millennium Stadium on May 21.
He is being plunged into an oxygen tank as United's physio team work around the clock to expedite his fitness.
Said Butler in the Yorkshire Evening Post: "I am absolutely gutted about it. These are the biggest games of my life as a Leeds United player and I can't play.
"I am devastated, but I'm focusing on backing the lads and helping them in any way I can. It's not about me, it's about the team, and we have a good enough squad of players here.
"I'll be in the dressing room and I will be a part of it. I hope the lads get through. I've always wanted to play for Leeds in front of 38,000 of our own fans, hopefully I'll get that chance if we get to the final. It's games like this that I came here for and I feel devastated."
The 33-year-old has rarely been troubled by injuries during his 15-year career, but felt a tweak in his calf at the end of last week.
Butler had a scan on the problem and his worst fears were confirmed as the injury was diagnosed as a torn calf.
"It's the worst timing possible," he added. "I had my head injury last season but apart from missing the odd game through suspension I've never really been a player who misses games.
"I've never really suffered tears or pulls, then this happens with three of the biggest games of my Leeds career to come. The physio team have been fantastic. They've worked around the clock, they've given me every opportunity and the treatment and attention is first class. There's nowhere better.
"But I'll get on with it. I want to help the lads in any way I can."
Gary Kelly will be handed the captain's armband for the double header.