Valera signed with the St. Louis Cardinals as an international free agent in May 2010. He made his professional debut with the Venezuelan Summer League Cardinals. He started 2014 with the Palm Beach Cardinals and was promoted to the Double-A Springfield Cardinals during the following season. Valera began 2016 with the Springfield Cardinals and was then promoted to the Memphis Redbirds, where he batted a combined.304 between both teams. The Cardinals added Valera to their 40-man roster after the 2016 season. On September 5, 2017 the St. Louis Cardinals recalled Valera from AAA Memphis Redbirds, and he made his major league debut the same night against the San Diego Padres starting at Second Base, batting 7th in the lineup. His first MLB hit, a single, came on September 7, 2017, against the San Diego Padres. Valera was designated for assignment on March 28, 2018.
Los Angeles Dodgers
On April 1, 2018, the Dodgers acquired Valera from the Cardinals in exchange for Johan Mieses. After beginning the season with the AAA Oklahoma City Dodgers, he was recalled to the majors on April 15.
The Orioles traded Valera to the San Francisco Giants in a cash transaction on January 5, 2019. Valera was designated for assignment on May 5, 2019, following the acquisition of Andrew Moore.
On May 12, 2019, the New York Yankees claimed Valera off of waivers. They assigned him to the Scranton/Wilkes-Barre RailRiders, and promoted him to the major leagues on July 7. On July 7, 2019, Valera made his Yankee debut and picked up a base hit in his first at-bat in a 2-1 loss to the Tampa Bay Rays.
Toronto Blue Jays
On September 20, 2019, he was selected off waivers by the Toronto Blue Jays from the New York Yankees and in his fifth game with the Blue Jays, on September 29, 2019, hit his first career Major League home run in an 8-3 victory over the Tampa Bay Rays at Rogers Centre in Toronto. Valera was designated for assignment on February 8, 2020.
San Diego Padres
On February 12, 2020, he was claimed off of waivers by the San Diego Padres from the Blue Jays.
Toronto Blue Jays (second stint)
On July 2, 2020, he was claimed off waivers by the Toronto Blue Jays from the Padres. On July 21, 2020, the Blue Jays placed Valera on the restricted list.