Sierra signed with the St. Louis Cardinals as an international free agent in 2012. He made his professional debut in 2013 with the Dominican Summer League Cardinals. In 2014, he played for the Gulf Coast Cardinals and was named the Cardinals Minor League Player of the Year after hitting .386 with two home runs and a .939 OPS. Prior to the 2016 season, Baseball America ranked him as the sixth-best prospect in the Cardinals\' system. He spent the 2016 season with the Peoria Chiefs of the Class A Midwest League, where he hit .307 with three home runs, 60 runs batted in (RBIs) and 31 stolen bases. The Cardinals added him to their 40-man roster after the season to protect him from being chosen in the Rule 5 Draft.\n', '

Sierra started the 2017 season with the Palm Beach Cardinals of the Class A-Advanced Florida State League. Because of injuries to José Martínez, Stephen Piscotty, and Dexter Fowler, the Cardinals called Sierra up to the majors on May 7, 2017, and started him in center field the same day in a 6−4 extra innings victory versus the Atlanta Braves. His first major league hit was a single in the sixth inning of his debut versus starting pitcher R.A. Dickey. After hitting safely in each of his seven games played (11-for-30), batting .367/.406/.367 .773 OPS with eight runs scored, the Cardinals optioned Sierra to the Springfield Cardinals of the Class AA Texas League. After his July 15 recall, he became the first Cardinals\' player to hit safely in all of his first nine games (July 16).\n', '

On December 14, 2017, the Cardinals traded Sierra, along with Sandy Alcántara, Zac Gallen, and Daniel Castano, to the Miami Marlins for Marcell Ozuna. MLB.com ranked Sierra as Miami\'s seventh best prospect going into the 2018 season. \n', '

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