Guilbeau spent most of his first three years at Alabama as a reliever and swingman, but moved into the rotation when he returned for his senior year, getting enough attention to be an attractive senior sign by the Nationals in the 10th round of the 2015 Draft. He began his pro career both starting and relieving, but it was a full-time move to the bullpen in 2018 that really got the lefty moving. He had reached Triple-A just a few weeks before he was sent to the Mariners at the Trade Deadline for Roenis Elias and made his big league debut just a couple of weeks after that.
Though “old” for a prospect, Guilbeau still has some upside on the mound. His stuff was ticking upwards when the Mariners acquired him and was missing both left- and right-handed bats while continuing to throw strikes. Guilbeau’s fastball-slider combination continues to improve, and he was sitting 95 mph and frequently touched 97-98 mph during his Major League debut, thrown with a ton of sink to get groundball outs. His mid-80s slider has good depth and his changeup has enough action to it for it to be a weapon against right-handed hitters and he threw it more often than his slider in the big leagues.
The two things Guilbeau will need to do in order to stick as a Major League reliever is to locate his changeup better, as big league hitters wouldn’t chase it out of the zone, and throw more strikes overall against left-handed hitters. The ingredients are there for him to get hitters out at the highest level; how much he can refine those things will determine his ultimate role.
Scouting grades: Fastball: 55 | Slider: 55 | Changeup: 45 | Control: 45 | Overall: 40
The Nationals' 10th-round pick in the 2015 Draft, Guilbeau was a senior signed out of Alabama who received a $25,000 bonus. He operated mostly as a swingman early in his pro career and had a strong first full season, albeit as a 23-year-old in the Class A South Atlantic League. A move up to Class A Advanced Potomac the next year was met with struggles, and the Nationals shifted him to the bullpen full-time when he repeated the level in 2018. He took quickly to the new role, pitching increasingly well as the season progressed before an eye-opening performance in the Arizona Fall League. He pitched his way from Double- to Triple-A in 2019 when he was sent to the Mariners for Roenis Elias.
After working in the low 90s during the regular season, Guilbeau's fastball was consistently 95-96 mph with improved late life in the Fall League. He has an above-average slider, thrown in the mid-80s with good depth, and also can mix in a fringy changeup. He has below-average control of all three pitches, though it stands to improve as he becomes more comfortable with his improved stuff out of the bullpen.
Guilbeau dominated same-sided hitters in the Carolina League with his fastball-breaking ball combo, holding them to a .184 average without allowing an extra-base hit in 54 plate appearances. Such success highlights Guilbeau's potential as a left-handed specialist, though he'll first have to improve his control as he navigates the upper Minors.