ST. LOUIS – The St. Louis Cardinals have added four prospects to their 40-man roster, protecting them from possible selection in December's Rule 5 draft.

Pitchers Austin Gomber and Derian Gonzalez and outfielders Oscar Mercado and Tyler O'Neill were placed on the 40-man roster ahead of Monday night's roster deadline.

Placing a player on the 40-man roster protects them from selection in the Rule 5 draft, in which teams can take unprotected players from other organizations. Only teams with open spots on their 40-man roster can make selections in the Rule 5 draft, and a player taken in the draft has to be added to the drafting team's 40-man roster. Monday's moves bring the Cardinals 40-man roster to 39. According to MLB.com's Jonathan Mayo, all of the Cardinals' top 30 prospects eligible for Rule 5 selection are now protected on the 40-man roster.

Gomber, a fourth-round selection by the Cardinals in 2014, was named the Cardinals Minor League co-Pitcher of the Year in 2015 and had 140 strikeouts in 143 innings for the Springfield Cardinals in 2017.

Gonzalez, 22, was signed as an international free agent in 2012. He had 15 starts at single-A Palm Beach this year.

Mercado is a 22-year-old drafted 57th overall by the Cardinals in 2013 as a shortstop. He has since been converted to an outfielder and was named an all-star for the Springfield Cardinals this year. He has 159 career steals in the minor leagues and participated in the Arizona Fall League, a prospect-rich showcase league.

O'Neill is the 22-year-old slugger the Cardinals acquired in a trade with the Seattle Mariners that sent Marco Gonzales to Seattle. O'Neill hit 31 home runs in 130 games between the triple-A affiliates of the two teams.