Sanmartin was initially signed by the Rangers as an international free agent on July 2, 2015, and spent three seasons in their organization before the trade.
In his professional debut, Sanmartin only pitched in six games out of the bullpen, allowing six earned runs on 12 hits and nine walks in 10 1/3 innings in the Dominican Summer League.
The 2016 season was a much better one for the left-handed pitcher, starting 12 games in the Dominican Summer League. He earned a 7-1 record with a 2.35 ERA and struck out 56 batters in 61 1/3 innings.
Sanmartin split the 2017 season between the Short Season-A Spokane Indians and the Class-A Hickory Crawdads. Between both teams, he earned a 7-2 record with a 2.45 ERA in 14 games, 11 starts.
The 2018 season was Sanmartin’s first season with the Yankees, and he started it with a brief one-game cameo with the Double-A Trenton Thunder. He allowed one earned run in that game and took a loss after pitching five innings.
He had two games in the NY-Penn League with Staten Island Yankees where he struck out 13 batters in 7 1/3 shutout innings.
The bulk of his playing time came in Class-A with the Charleston RiverDogs where he started eight games and earned a 2-6 record. He struck out 31 batters in 43 innings and had a 4.19 ERA. He ended his season in High-A with the Tampa Tarpons where he struck out 10 in 12 innings allowing just one unearned run.
Sanmartin is a soft-tossing left-handed pitcher that will likely move into the bullpen full-time as he moves up the ladder in the minors. His fastball is 89-92 miles per hour, and he backs it up with a slider and a changeup. He is a control pitcher that attacks the strike zone.