When Johnny Knox went down in the game against Seattle with a back-bending hit that still makes my skin crawl every time it's mentioned, it took away one of the better receiving options on the team for the rest of the season and, at the time, put his career in jeopardy. Since then, his outlook has changed, but as Sam reported earlier today, it's entirely possible Knox doesn't play in 2012, with an eye on rehabbing fully and returning for 2013.
Though it might not be the best deal for him personally - 2012 would be his contract year, and thus his last chance to play himself into a better contract - the Bears don't need to force him back, and he doesn't need to force himself back.
The Bears' side of this is simple - they no longer need to rely on the streaky, somewhat inconsistent receiver who's been their top receiver option since his arrival in 2008. They completely overhauled the position by trading for Brandon Marshall and drafting Alshon Jeffery. In the second area where he contributes, kick returns, the Bears went after a couple other options there by adding Eric Weems, a very solid returner from Atlanta, and drafting Greg McCoy.
Knox's skill set may be replicated on the current roster, but Knox still has a place on the roster provided the team wants him back. He'll still earn an improvement of a contract in the offseason provided his back is sound, and if a team isn't convinced, Knox can play a year on a small contract and with a good performance, he can earn back a decent contract in 2014.
If the Bears hadn't played to improve the receiving and return corps in the way they did, they might have tried to push Knox back, and Knox might have felt pressure to push himself back (we don't know for sure, but as the primary guy in one of those roles, it's possible). Suppose he comes back with a wobbly back and it doesn't hold up. It puts the team in a worse position, having no backup plan with their top receiver down, and Knox likely ends his career.
Since the Bears did pick up Marshall, Jeffery and Devin Thomas, as well as Weems and McCoy, though, they don't need Knox to come back in any rush and play injured, or only part-way rehabbed. If Knox is done for 2012, and the Bears wanted to retain his services, his current contract pays him $1.26 million in 2012. The Bears could offer him a one-year extension for around a million, with an eye on extending him with a decent multi-year deal should he prove his back can handle the rigors of NFL life. He probably won't be able to pick up a multi-year deal immediately, but by playing himself into one when his back is fully healthy, it's better than the career-ending alternative.