PHOENIX - The value of a shutdown bullpen was on display no more so than in this year's playoffs. The World Series champion Kansas City Royals routinely shortened games with their formidable relief corps, led by an unhittable closer in Wade Davis.

As the Phillies embark on the second offseason of a rebuilding process that will span several years, their bullpen is far from set for the long term. Among last year's group, arguably only closer Ken Giles, 25, and Elvis Araujo, 24, can be looked upon as building blocks for the future.

Enter Tom Windle, the less-heralded of the two pitching prospects the Phillies acquired in December's trade of franchise icon Jimmy Rollins. A lack of an effective third pitch and a deluge of walks stalled his career as a starter, but the 6-foot-4 lefthander has found success since he was converted to a relief role in June.

Windle's progress has continued the last 31/2 weeks in the prospect-laden Arizona Fall League, where he has further settled into his bullpen role. He has made seven appearances for the Glendale Desert Dogs, logging seven innings, six of which were scoreless. His fastball command no longer eludes him.

"I've thrown enough in relief right now to where I'm starting to get pretty comfortable," Windle, 23, said last week after a game at Camelback Ranch-Glendale, spring training home to his former organization, the Los Angeles Dodgers.

Windle, a second-round draft pick of the Dodgers in 2013, has seen his velocity increase a tick or two in Arizona. His fastball has been clocked between 93 and 96 m.p.h. in the fall league, an encouraging development he attributes to time spent with the Phillies pitching coordinators in the recent instructional league. They worked with the pitcher to loosen his arm action, and he has felt the results.

Windle complements his fastball with a slider, his best pitch dating to his days at the University of Minnesota. Since converting to a relief role, he has ceased throwing his change-up, long his weakest offering, but still works on the pitch when playing catch with teammates. He doesn't want to abandon it altogether, but it is very much a work in progress.

"If I can get guys out with [two pitches] right now," he said, "I'm just going to try to perfect those as much as I can."

Windle opened last season as the No. 4 starter in a ballyhooed double-A Reading rotation headlined by Aaron Nola. When the organization moved him to the bullpen, Windle had a 5.35 ERA over 14 starts with only three more strikeouts (43) than walks (40). Only three of his outings had reached six innings.

But in relief, Windle posted a 1.69 ERA over 262/3 innings to cap Reading's regular season.

"I needed it," Windle said of the transition. "Result-wise, I wasn't pitching good. Numbers wise, it wasn't good. It just helped clear my mind, and it gave me a change."

With only two weeks of the fall league remaining, Windle has a 2.67 ERA. Although the Phillies typically don't reveal which prospects are invited to major-league spring training until closer to February, Windle could be one of them. His new role should help him reach Citizens Bank Park at some point next season.

"That's up to the Phillies," Windle said. "I think the biggest thing with anyone is if you show that you can pitch at the major-league level, they're going to move you whether you're a starter or a reliever. I think with myself, right now, being a reliever, I've shown a little more of that."

kaplanj@phillynews.com

@jakemkaplan