Forshaw has been out since September with a troublesome hip problem, and the midfielder is now going to require surgery before he can come back.
Leeds have taken the option as a last resort, but have no other choice but to allow Forshaw to travel to America to undergo the procedure.
Forshaw now want to be fit to feature for Leeds until next season.
And the midfielder said: "I really want to get back fit and play at Elland Road and I'm devastated to be in the situation I'm in, but unfortunately I do need to have surgery if I ever want to get back to 100 per cent fitness.
"I want to take this opportunity to thank the medical staff at the club, my teammate, but most of all the fans who have been really supportive - especially when I've seen them around Leeds.
"It's been the worst period of my career.”
Forshaw’s absence is a big blow for Leeds, as the £4.5 million (BBC Sport) man could have had a key role to play for Marcelo Bielsa’s side.
Forshaw made a fairly bright start to the campaign at Elland Road before injury struck.
Leeds will now have to rely on their other midfield options to get them over the line in the race for promotion.
Leeds have clung on to second spot this week, but they have a clutch of teams breathing down their necks in the race for automatic promotion now.
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