The Royals will not re-sign Johnny Cueto. He will make a boatload of money elsewhere. With the rotation currently shaping up to be Yordano Ventura, Edinson Volquez, Danny Duffy, and Kris Medlen, the Royals still need a fifth starter. Jason Vargas will, I'm sure, be eligible to return at some point during the season, but who knows how effective he will be. Duffy was moved to the bullpen in favor of other starters, including Chris Young, during the playoffs. Ventura had some rough stretches last year. Chris Young may or may not be available back on a short deal. There are not really any prospects who are close. The point is that the Royals could use more pitching. Injuries and ineffectiveness happen.
The Royals won't be in the market for David Price, Zack Greinke, or any of the other top-tier free agent starting pitchers. They'll be ...active, I guess...in the mid-tier free agent market. Left-hander Wei-Yin Chen fits squarely in that mid-tier free agent market. He also fits fairly well what the Royals do. The question is will the Royals spend dollars on pitching.
Chen is a contact manager
Part of the reason Chen will not be a top-tier option is because he does not have exceptional strikeout rates. Greinke does not strike out a lot of guys either, but he has other qualities that cause him to be so valuable. Indeed, one does not need elite strikeout rates to be an elite pitcher, but it makes it so much easier. Chen, then, has a lower floor than a guy like Price. Fairly simple.
However, Chen has some of those other qualities that make a pitcher good. Over 706.2 innings in his career, he has outperformed his FIP and xFIP by about 0.4 - a relatively significant amount. He has done so despite having only a slightly-lower-than-average career BABIP allowed (.290) and being a fly-ball pitcher in a home park not friendly to fly-ball pitchers (Oriole Park at Camden Yards). Chen's homer rate reflects that environment, but his actual runs allowed have not suffered as much for it.
There are two reasons why - one, like Chris Young, he induces a fair number of popups. His 14 percent infield-fly ball rate in 2015 (by FanGraphs) was well above the MLB starting pitcher average of 9.5 percent. Two, as he has accrued experience in the league, he appears to have become a more skilled contact manager in general. His soft-hit rate allowed and hard-hit rate allowed were both better than MLB starting pitcher average in 2015. Those numbers are also trending positively. Being a fly ball pitcher who induces weaker contact, Chen fits with what the Royals' strengths are.
Chen is consistent
In his four years in the league, Chen has ranged from 2.0 fWAR to 2.8 fWAR. That's pretty consistent considering that decimal points of fWAR are not particularly significant. In addition, that 2.0 fWAR season was over only 137 innings; Chen has pitched between 185 and 193 innings the other three seasons. His BABIP has ranged between .290 and .305 the past three seasons. He'll give fairly good quality innings.
His batted ball mix has stayed fairly stable as well. Line drives are known to fluctuate year to year, but Chen's only line drive rate blip (24.6%) was in 2013. He's been around 20-21 percent the other seasons, which is close to league average. His infield-fly ball rate has remained in the double digits each year. His HR/FB has ranged from 9.9 to 12.3 percent. There's not much variance.
Chen's stuff also follows the theme here. For the past three years, his average four-seam fastball velocity is basically unchanged. He relies on that pitch more than 50 percent of the time, so it's an important one for his success. He can throw it for strikes, and it is the pitch with which he generates the most popups. Even though he is a lefty, he is not afraid to bust righties inside with that fastball, which, again, is where he generates the most popups.
Unfortunately, the consistency cuts both ways. Chen's platoon split has been fairly stable through his career - righties can mash him a bit. Chen uses a split-finger against righties, but the pitch does not have a huge velocity gap from his four-seam fastball (~8.6mph in 2015), and according to Brooks Baseball batters rarely whiff against the pitch (6.7 percent in 2015). The pitch does drop and fade more than the four-seam fastball, but again the difference is not large. When compared to the sinker, the splitter sets itself apart even less. The splitter is hittable.
Aside from the aforementioned platoon split, Chen has two other things going against him. One is his age. Chen will turn 31 during the 2016 season, and giving multiple years to slightly older pitchers, or pitchers in general, is something teams like to avoid, except for "Ace"-type pitchers or younger guys like Ventura. The Royals already have a mid-to-lower tier free agent signee in Jason Vargas, so they may be hesitant to sign another.
Yes, I did mention that Chen is in the mid-tier of free agents, but mid-tier does not equal cheap anymore. According to the FanGraphs' crowd-sourcing project, Chen is predicted for a 4-year, $52 million contract. Dave Cameron predicted a 4-year, $64 million contract. That's an average annual value of $13 million to $16 million. At Dave's prediction, Chen is double the price of Vargas for the same number of years. Is Chen really twice as good? Probably not. If the Royals signed anyone for that amount of money, it would more likely be Ben Zobrist. Right?
Chen also was given a qualifying offer, so signing him means giving up a draft pick. Should Alex Gordon depart though, and the Royals signed Chen, they would end up with a compensation round pick, which is still worse than their current first-round pick. That's not an exciting consolation prize for losing Gordon in this hypothetical, but it's something. Of course, the opportunity cost is not signing any free agent with a qualifying offer attached, in which scenario the Royals would end up with two draft picks.
Overall, Chen has his warts, but he is pretty much a known commodity. His floor and ceiling are fairly predictable, and he fits the Royals' contact-and-defense strategy. He would provide some stability, but it wouldn't be cheap. We'll have to wait to see how the Royals spend their World Series dollars.