The Los Angeles Dodgers selected Valentín in the first round of the 2012 MLB Draft out of the Puerto Rico Baseball Academy and he signed, forgoing his commitment to play college baseball at LSU. The Dodgers assigned him to the Arizona League Dodgers to begin his professional career that year and he hit.211 with two home runs and 18 RBIs in 43 games. He split 2013 between the Great Lakes Loons and the Ogden Raptors, and he hit.264 with four home runs and 29 RBIs in 95 games. He began 2014 with Great Lakes where he hit.280 with seven home runs and 47 RBIs in 108 games. The Dodgers traded Valentín to the Philadelphia Phillies on August 16, 2014, as a player to be named later in a deal that sent Roberto Hernández to the Dodgers. The Phillies assigned him to the Clearwater Threshers where he batted.205 in 12 games. He began 2015 with Clearwater. On April 11, 2015, he was placed on the restricted list and suspended indefinitely following his arrest for a domestic violence incident. The Phillies added him to their 40-man roster after the 2016 season. After he was activated off the restricted list, he returned to Clearwater, where he spent the whole season, posting a.273 batting average with one home run and 14 RBIs in 31 games. In 2016, he spent time with both the Reading Fightin Phils and the Lehigh Valley IronPigs, batting.269 with nine home runs, 52 RBIs and a.736 OPS in 125 combined games between both teams. He spent 2017 with Lehigh Valley where he batted.229 in 29 games before a separated shoulder caused him to miss the remainder of the season. He began 2018 with Lehigh Valley.
Philadelphia Phillies
Valentín was promoted to the majors for the first time on April 29, 2018. He recorded his first career Major League hit on May 7 against the San Francisco Giants. He hit his first career home run and recorded his first multi-hit game on June 13 against the Colorado Rockies. Valentín made an appearance as a pitcher in during a June 15 game against the Milwaukee Brewers, surrendering three runs over one inning while recording one strikeout. He was designated for assignment on September 1, 2018. Valentín elected free agency on November 2, 2018.
His father, José Valentín, and uncle, Javier Valentín played in Major League Baseball. His first child, a daughter named Ximena, was born on June 25, 2018.
