Ultimately no one -- not the Mets, not manager Mickey Callaway, not pitching coach Dave Eiland and not Matt Harvey himself -- knows how this new chapter in Harvey's career will turn out.
That Harvey has ended up in the bullpen in New York is somewhat surprising, and it's arguable he deserved a shot at one more start given that he finished strong in his last outing against Atlanta and looked good when he toed the rubber against Philadelphia in his first start of the year, pitching with much more conviction than we saw last season.
With that said, Harvey has a 6.57 ERA over the past two seasons, so the decision is certainly defensible, and he will be available out of the bullpen tonight against St. Louis for the first time. And if he embraces his new role, I see him helping the club in relief and pitching his way back into the starting rotation.
It's almost certain the Mets prefer that outcome has well. The health of their staff is still a question mark, especially with Zack Wheeler and Steven Matz, who haven't yet shown they can stay healthy and pitch effectively for a whole season. That uncertainty leaves a window open for Harvey, but he needs to find some of the stuff that made him so dominant in the past.
How relief could help Harvey
Harvey's average fastball velocity has fallen from 96.5 mph in 2015 to a career-low 92.6 mph so far in 2018. His slider has lacked consistency, and his changeup has lacked finish -- he's allowed a .385 batting average and a .692 slugging percentage on that pitch this year.
Pitching out of the bullpen should help Harvey's stuff "play up." We should see an increase in his fastball velocity -- which is typical when guys move to the 'pen -- and a better, harder slider in these shorter relief stints. Harvey's changeup might pose more of a challenge, as it's a tougher pitch to find the feel for over a brief relief outing.
But most importantly, Harvey needs to use this time as a reliever to find his command within the zone. His walk rate is low (4.5 percent), which is encouraging, but without elite velocity, he needs to hit the corners more, and this has been an issue for him since he had surgery to alleviate Thoracic outlet syndrome in June 2016. Returning from that condition is still an uneven proposition, and pitchers who have made successful returns to the rotation after pitching in the bullpen -- specifically, the Royals' Danny Duffy and the Indians' Danny Salazar, Carlos Carrasco and Trevor Bauer -- haven't had the same type of injuries that Harvey has.
Those four test cases are relevant here, though, as Callaway worked with Salazar, Carrasco and Bauer in Cleveland, and Eiland with Duffy in K.C. Those guys know how the bullpen can help starters revive their careers, and Callaway and Eiland are presumably selling Harvey on that fact. He's set for free agency this winter, and he would certainly like to hit the market being able to sell himself as a starter.
Of course, the Mets' bullpen isn't just a place for Harvey to work on regaining his old form. He'll be having an actual impact on whether the team wins games.

Harvey's bullpen role
So what exactly will Harvey's relief role be? The Mets do seem to have need for a right-handed bridge to closer Jeurys Familia, with AJ Ramos struggling of late and free-agent setup-man signing Anthony Swarzak likely still out at least a few more weeks with a sore left oblique. But it's not clear that those high-leverage innings would go to Harvey.
For one thing, even though Ramos has a 4.00 ERA this season (and has had a poor 2018 by some advanced metrics, like his 6.44 xFIP), he has a track record of success as a closer, which allows him more rope in a late-inning role. He's held lefties to a .196 batting average in his career, and righties to a .198 batting average, consistency that should convince Callaway to give Ramos more full innings of work.
Robert Gsellman will also see more action in the high-leveraged innings if he continues his early-season success -- Gsellman has a 2.19 ERA and 18 strikeouts in 12 1/3 innings over his 10 relief outings.
That would leave Harvey to pitch in the lower-leverage innings for the time being, in a long man or swingman-type role, something more akin to the way the club has been using Seth Lugo and Paul Sewald. Figuring out how to warm up and establish his pitches from the beginning might take an outing or two, but once he gets used to that, if Harvey can throw his pitches with conviction in his relief outings, that will be a good sign for the Mets.
How will this play out?
Pitching out of the bullpen temporarily should give Harvey the opportunity he needs to get back on track, even though he's never pitched in relief as a professional other than one unique outing last September when he piggybacked Noah Syndergaard in Thor's return from a right lat injury. Harvey is capable of adapting to the role and taking advantage of it.
Right now, the Mets have Syndergaard and Jacob deGrom locked into the top two spots of their rotation, followed by Matz, Wheeler and Jason Vargas, who is set to return vs. the Padres from an injury to his non-pitching hand on Saturday. Those final three spots are from sure things, and Harvey could easily make his way back into the rotation if he can gain some confidence in the 'pen, especially if he thrives in multi-inning stints.
Ultimately, I expect Harvey's move to the bullpen to be a temporary one. He should eventually return to the Mets' rotation -- and not only that, pitch effectively, even if not as an ace anymore, at least as a back-end starter.