All pitchers are fine until they aren't. Health can be snatched away by a nasty coalition of the arm, shoulder, and baseball gods in an instant (note to Royals - more chicken sacrifices). When health is snatched away, that can present an opportunity for a team.
For example, Henderson Alvarez was good and healthy-ish (all non-injured pitchers are only healthy-ish, really). Then he wasn't. The Athletics got him on a one-year, $4.25M deal. Another pitcher in this vein is Cliff Lee.
Lee last pitched in 2014. Making 13 starts for the Phillies, Lee threw 81.1 innings of 3.65 / 2.96 / 3.01 ERA / FIP / xFIP ball. His strikeout rate was definitely down, and his BABIP was definitely up, but given the current Royals rotation I would take that performance.
Let's be clear about one thing first - Lee is not a pitcher about which we need to consider how the Royals home park and defense will help. Those things will help any pitcher. However, when we normally look for "undervalued" pitchers, we are looking for those whose disadvantages get canceled out by the Royals' advantages. Guys like Chris Young.
Cliff Lee is not that kind of pitcher. He already limits home runs. He is not much of a fly ball pitcher anymore. He is a control artist who also does a good job of limiting damage on contact; he does not need a defense to do that for him. Because of this, there will be other teams interested. Lee was (is?) a good pitcher, but I am not interested in a bidding war for a guy who sat out 2015 due to injury and is 37 years old.
Having said that, I would be glad to have Lee on a one-year bounceback deal.
Going off his last full season of data, 2013, Lee's pitch mix includes a sinker, cutter, changeup, and curveball (in that order). Against lefties, he'll stay almost exclusively sinker/cutter until he gets ahead in the count or gets two strikes, after which he introduces his curveball. Against righties, he uses the exact same strategy except with the inclusion of the changeup in all counts.
He throws the sinker right over the plate. He'll go back foot against righties with the cutter and low and away against lefties, which is the same location. He buries the curveball under the zone in the same area. He'll put the changeup low and away to righties.
Overall, it's a fairly predictable pitch mix and location mix. Again, where Lee sets himself apart is in his control. He's like Bartolo Colon in his propensity to throw strikes, except he is much harder to hit. In 2013, his contact rate, especially outside the zone, was much lower than Bartolo's. Lee gets his control from his delivery. Though it has some imperfections, he can repeat his delivery better than pretty much anyone. Repetition is the most important factor in control, and Lee has it in spades.
So that's Lee in a nutshell. Mike Axisa noted that Lee would be a good fit for the Royals. Chris Young and Kris Medlen have durability questions, and Danny Duffy has bounced back and forth between the rotation and the bullpen. Edinson Volquez and Yordano Ventura seem to be the only "sure bets" for the rotation at this point.
Should the Royals sign Lee, however, someone would have to get bumped. That would likely be Chris Young, as he excelled in the swingman role last year. It would also help keep him fresh.
The Royals are definitely in the market for a pitcher. They've been connected to Scott Kazmir, Yovani Gallardo, and Ian Kennedy to name a few. Wei-Yin Chen is still out there. Kazmir signed a three-year deal with an optout, but the others are still available. Those guys would require multi-year deals, but Lee almost certainly would not unless the bidding gets craaaaazzzzyyyyy.
Another wrinkle in this whole shebang is that Lee's agent said Lee would pitch only in the "perfect fit". Who knows what that means, but the Royals are competitive. They've been to the championship two years in a row and face a weak division. Those are probably criteria in the perfect fit definition.
So Lee at his most recent healthy state was a good pitcher, and he could slot right in with the Royals. What remains is if Lee can deal with his elbow injury and still pitch. He tore a flexor tendon in his pitching elbow and elected to go the rest/rehab route instead of surgery. He has been medically cleared to throw.
The Royals recently had success with Ryan Madson, who had a long layoff from pitching before having a good season in the bullpen last year. The Royals are attempting it again with Kris Medlen, who did not pitch at all in 2014 but did some good things in 2015. They have a pretty good track record with health overall. It is hard to declare with certainty that the Royals would be more successful at keeping Lee healthy and effective than others teams without really analyzing more data, but their track record with days on the DL is a point in their favor.
If Lee wants a successful comeback with a competitive team, he won't find a much better fit than the Royals.