Granillo made his professional debut with the Single-A Palm Beach Cardinals with whom he pitched to a 1.50 ERA over 18 innings. He opened the 2022 season with Palm Beach and was promoted to the High-A Peoria Chiefs and Double-A Springfield Cardinals throughout the season, going 4–6 with a 4.13 ERA and 82 strikeouts over 52+1⁄3 innings pitched between the three clubs. Granillo returned to Springfield to open the 2023 season and was promoted to the Triple-A Memphis Redbirds in mid-August. Over 53 relief appearances between the two teams, he went 3–4 with a 4.74 ERA, 89 strikeouts, and 14 saves over 68+1⁄3 innings. He was selected to participate in the Arizona Fall League with the Scottsdale Scorpions after the season. Granillo returned to Springfield to open the 2024 season and was promoted to Memphis in mid-May. Over 48 relief appearances between both clubs, Granillo posted an 8–2 record and 3.88 ERA with 80 strikeouts over 65 innings.
Granillo was assigned to Memphis to open the 2025 season. Over 18 relief appearances and 29+2⁄3 innings prior to his MLB promotion, he posted a 1.82 ERA and 46 strikeouts. On June 12, 2025, the Cardinals selected his contract and promoted him to the major leagues. He made his MLB debut that night versus the Milwaukee Brewers at American Family Field, pitching 2+2⁄3 scoreless innings with two strikeouts. On June 19, Granillo appeared in both games of a doubleheader against the Chicago White Sox; he recorded his first career win in the first game, and logged his first career save in the second game. He was optioned to Memphis and recalled to St. Louis various times during the season. In total, Granillo appeared in 14 games for the Cardinals and went 1–0 with a 4.71 ERA and 18 strikeouts over 21 innings.
Washington Nationals
On February 10, 2026, Granillo was traded to the Washington Nationals in exchange for George Soriano. He was named to Washington's Opening Day roster, the first of his MLB career. He made eight appearances for Washington, recording a 9.64 ERA with three strikeouts across 9+1⁄3 innings pitched. He also made 25 relief appearances for the Triple-A Rochester Red Wings, with whom he had a 6.82 ERA. On July 4, Granillo was designated for assignment by the Nationals.
Detroit Tigers
On July 11, 2026, Granillo was claimed off waivers by the Detroit Tigers and optioned to the Triple-A Toledo Mud Hens. He was designated for assignment and removed from Detroit's 40-man roster on July 31. On August 4, Granillo cleared waivers and was sent outright to Toledo. He pitched in 13 games for Toledo and had a 3–0 record, a 6.89 ERA and 19 strikeouts over 15+2⁄3 innings.
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