As to what would happen, look back to that linebacker that was kept on the team for several seasons while he healed. At one point he was not expected to be able to walk normally. Sounds like your simple as that is not so simple. The kids name was Sean Spence in case you forgot.
As to Antonio he has out played his contract so far. Not saying he was poorly compensated but he has been a good player the entire time he was here and has been continually improving. This maybe nothing more than a negotiation ploy by his agent. I have not looked at the salaries of other wide outs but suspect they are paid more than Antonio and less productive. By comparison Jason Worilds was paid 9 million last year for production that was no where near as important to the team or anywhere close to the best in the league.
I can understand these guys wanting to make the best of their productive years salary wise as most of them at best have 10 or 12 years to earn most all the money they will make for the rest of their lives. They also run the risk of never being able to play or do anything again.
The Steelers have a couple of options one of which is very time sensitive.
Trade to another team for their top picks for maybe 2 years or more if they can get it as well as players. Maybe a top left tackle, and a number one pick or more depending on what the market will yield.
Let him hold out and force him to honor his contract.
Extend his contract and run the risk of doing the same thing in a few years.
Do a completely new contract.
The first thing I would do at this point is see what offers are available for trade and not be quiet about it. It would send a message to the player and his agent. Imagine how well he will do in Cleveland or Oakland or Buffalo or Arizona for example. That would allow the trade option to be done prior to the draft and the draft to be used to fill in some spots. Also if the trade option is not used it may allow this situation to be taken care of prior to the draft.