There is a case to be made for the Rays moving starting left-handed pitcher Drew Smyly to the bullpen. Let's run through it.
The bullpen was shaky in 2016
The bullpen (along with the catcher position) is probably the Rays highest priority off-season upgrade. The 2016 relief corps combined to have the highest HR/9 rate, highest HR/FB rate, 3rd highest BB/9 rate, worst FIP, 3rd worst xFIP, 5th worst ERA, and worst fWAR total in the league. Okay that's not shaky, that's terrible.
The bullpen could improve in 2017 by not having Steve Geltz, Dana Eveland or Kevin Jepsen in it. They combined for 69 innings of some of the worst pitching in one of the worst bullpens. Even replacement level players taking their spots will improve the bullpen, but that's not even close to good enough. To move the needle the Rays will need to add a real weapon to the pen.
Drew Smyly can be that weapon.
He would join Xavier Cedeno as another reliable lefty in the pen. (Enny Romero is another lefty option, but he's hardly reliable). This move would probably put Romero is the lowest leverage situations, which is probably for the best.
Smyly has had success as a reliever
In 2013 Smyly came out of the Detroit Tigers bullpen the entire year. He excelled in that role to the tune of a 2.37 ERA, 2.31 FIP and 2.99 xFIP over 76.0 IP in 63 appearances. That kind of production would have been 2nd best to Alex Colome in the 2016 Rays bullpen. There is an explanation for that success, and it's a process the Rays can replicate.
Smyly has significant L/R splits. He has a career 2.76 FIP and 2.95 xFIP vs LHH and a 4.25 FIP and 4.27 xFIP vs RHH, meaning he is far better vs. lefties than righties. As a reliever, he was able to enter game situations that took advantage of his L/R splits in a way that can't be replicated in starts, especially as the opposing team knows his vulnerabilities and stacks their lineup with righties.
The chart above shows that in 2013, his full year of relief, he threw a higher percentage of his pitches to lefties than ever, which is the side he has had far better results against over his career. Smyly doesn't have to be a LOOGY, but he would be a lights out reliever if he was frequently brought into game situations where 2 of the next 3, or 4 of the next 6 batters due up were lefties. That process would maximize the chances of replicating Smyly's 2013 season.
Smyly struggles after first time through order
Not only does Smyly struggle against righties; he also struggles getting through a lineup after the first time. This is true for pretty much every pitcher, but even moreso for Smyly. The average pitcher allows a .345 wOBA the first time through the order, and then .354 the next, an increase of 9 points. Over his career, Smyly has averaged a .285 wOBA the first time through and then a .349 woBA the second time as a starter, for an increase of 64 points. This shows Smyly is fantastic to start but is pretty much an average pitcher after that.
The Rays have starting pitching depth
Even if they move Smyly to the bullpen, the Rays will still have Chris Archer, Alex Cobb, Jake Odorizzi, Blake Snell, Erasmo Ramirez, Matt Andriese and Chase Whitley all at the major league level. The minors are also stacked with Jacob Faria, Jaime Schultz and Taylor Guerrieri likely waiting in AAA Durham for their chance, and top prospect Brent Honeywell will probably force his way into the majors by the end of 2017 with the way he's performing.
The upshot: there is no shortage of starting pitching in the Rays organization, which should give the Rays the flexibility to use their pitchers in creative ways. The Rays are looking to trade a starter this off season, but they will still have the depth to move Smyly to the pen as long as they don't trade away more than one.
Assuming the Rays trade one starter (Odorizzi in this example), this move would have the Rays opening day pitching looking something like:
Rotation: Archer, Cobb, Snell, Andriese, Whitley/Erasmo
Bullpen: Colome, Smyly, Cedeno, Boxberger, Whitley/Erasmo, Romero, Durham Shuttle
At an expected arbitration price of $6.9M and coming off a 2.0 fWAR season, a move to the bullpen doesn't seem like the Rays way, but it could very well be what the Rays need right now to win.