Isaac Mattson
- Winter Plan bullpen · 50 IP · 0.4 WAR in the role
- Injured list 2026: Right hip impingement (Sep 8–Sep 28)
This winter
What the Winter Plan does with Mattson: its decisions feed, the dated moves and the briefs
The plan keeps him: the bullpen · 50 IP · 0.4 WAR in the role.
Options left: unknown (Keystone can’t estimate it from public data).
- 2027 starter hook and bullpen rolesdue Spring training · medium confidence
Rotation (Winter Plan): Paul Skenes, Jared Jones, Braxton Ashcraft, Bubba Chandler and Mitch Keller. Two times through for Mitch Keller once his medicals clear. Let Braxton Ashcraft face the order a third time (a toss-up). Let Paul Skenes face it too, pending the review of his velocity drop. Jared Jones: a 22-batter leash (about 81 pitches) after his arm injury, not an open third time; past it, decide by matchup (a toss-up). Give Bubba Chandler development innings on a 22-batter leash (about 82 pitches), once through the top of the order a third time, at about 1.0 ± 1.2 runs against a two-times-through hook (a toss-up); the goal is first two passes at .306 xwOBA or better, not a third-time skill, pending the review of his velocity drop. Ladder: Mason Montgomery and Yohan Ramírez in the late innings (within noise of each other); Evan Sisk and Luke Weaver setting up (within noise of each other). Move Evan Sisk into the leverage ladder. Move Isaac Mattson out of it.
Development · trends & plan
Tracked levels only. Monthly values; gaps mark the offseason. The number beside each line is its last month, with that month’s sample (faded under 50 PA or 150 pitches), not the season.
Plan for 2027
- 1 Health medical first Medical clearance before any development work
Right hip impingement (15-day IL, Sep 8–Sep 28, 2026); 0 games after it. Every other goal on this plan starts after the medical staff clears him.
Health & Workload
Each goal is checked when his sample can tell the target from noise: the 2027 All-Star break if it can by then, else season’s end or the sample it needs; if no single season can, it’s a lean. Coach card
Trends · MLB
Clear changes
- ▼ Stuff 101 → 91 −10 2025 → 2026 · Triple-A
Pitching · 2026 percentiles
vs. MLB pitchers with 500+ pitches · 728 pitches
Arsenal & Pitch Lab
Movement normalized to arm side · Keystone Stuff by pitch
| Pitch | Use | Velo | IVB | HB | Stuff | Loc | Whiff | xRV/100 |
|---|---|---|---|---|---|---|---|---|
| Four-seam70% use | 70% | 94.2 | 18.1 | +9.3 | 99 | 94 | 19% | −0.68 |
| Sweeper10% use | 10% | 82.9 | −0.4 | −8.8 | 96 | 109 | 28% | −1.94 |
| Changeup10% use | 10% | 86.0 | 5.5 | +9.2 | 98 | 76 | 18% | −1.50 |
| Slider9% use | 9% | 84.9 | 3.0 | −2.4 | 96 | 105 | 28% | −0.41 |
No mix change or new pitch clears its bar. The arsenal is efficient for its shapes.
Most similar pitchers
Pitches like Mattson: same hand, shape, slot and mix
| Similarity | Player | Stuff | ’26 WAR | ’27 proj | 2027 |
|---|---|---|---|---|---|
| 48 | Gordon Graceffo STL · ’00 | 91 | 0.6 | 0.6 | Pre-arb |
| 47 | Kyle Finnegan DET · ’91 | 106 | −0.8 | 0.0 | Signed $8.0M |
| 47 | Zach Agnos COL · ’00 | 87 | 0.1 | 0.1 | — |
| 46 | Tobias Myers NYM · ’98 | 101 | 0.2 | 0.5 | Pre-arb |
| 42 | Ben Brown CHC · ’99 | 100 | 2.0 | 1.9 | Arb |
Same-handed pitchers nearest on fastball velocity and shape, arm angle, extension, pitch mix, secondary shapes, Stuff, whiff rate and workload. 2026 regulars only.
Pitch locations · 2026
Where each pitch finishes, by batter side
Where each pitch finishes, catcher’s view. The bar marks the batter’s side; the box is an average strike zone. Pitches with 40+ thrown to this side.
Contract & projection
Club control (RosterResource) · Keystone projected WAR
* Estimated: RosterResource has no contract row for him, so control through 2027 estimated from his MLB debut (2021-05-07): six years of service at the earliest; he isn’t in the contract data, and time in the minors would add seasons; pre-arbitration years at the minimum, arbitration years from Keystone’s arbitration model. Contract Studio →
Minor leagues
MLB Stats API · public box-score data
| SeasonLevel | Level | Team | Age | G | PA / IP | PA / IP ·Line |
|---|---|---|---|---|---|---|
| 2026AAA | AAA | Indianapolis Indians | 30 | 15 | 20.1 IP | 20.1 IP · 1.33 ERA · 29.6% K · 11.1% BB |
| 2025AAA | AAA | Indianapolis Indians | 29 | 19 | 21 IP | 21 IP · 2.57 ERA · 29.8% K · 7.1% BB |