The Cardinals' 40-man roster is full.
St. Louis previously dropped Mitch Harris, Alberto Rosario and Mike Ohlman off its roster in October, then designated Dean Kiekhefer and Jeremy Hazelbaker for assignment earlier this month. Both were claimed on waivers.
Sierra, the Cards' No. 6 prospect according to MLBPipeline.com, was an obvious selection. The 20-year-old future center fielder hit .307/.335/.395 with 31 steals for Class A Peoria this year. Signed out of the Dominican Republic in 2012, Sierra was named the Cardinals' Minor League Player of the Year in 2014.