ST. LOUIS -- The Cardinals added three of their top 18 prospects -- outfielder Tyler O'Neill (No. 4), left-hander Austin Gomber (No. 15) and outfielder Oscar Mercado (No. 18) -- as well as right-hander Derian Gonzalez to the 40-man roster on Monday in order to protect all four from being exposed in next month's Rule 5 Draft. Clubs had until 7 p.m. CT to protect Rule 5-eligible players.
With the moves, the Cardinals now have one opening remaining on their 40-man roster. The Cardinals cleared the necessary roster space for these additions earlier this month with a series of moves, including the release of Trevor Rosenthal .
• About the Rule 5 Draft
Eligibility for the Rule 5 Draft is dependent upon how old a player was when he signed with the organization. Players who signed at the age of 18 or younger become eligible after five seasons. Those who were 19 or older have four years before they must be protected. This year's Rule 5 Draft is scheduled for Dec. 14, the final day of baseball's Winter Meetings in Lake Buena Vista, Fla.
While the Cardinals benefited from the Rule 5 Draft two years ago with their selection of right-hander Matt Bowman, they've also recently experienced some losses. The Padres plucked an unprotected Luis Perdomo from the Cards in 2015 and then dipped into St. Louis' system again last year to take Allen Cordoba.
The Padres kept both on their 25-man roster the following season to secure the permanent rights to the player.
The moves caught the Cardinals by surprise given that neither player had ascended beyond the Class A level at the time of their respective selections. But as a club in the midst of a rebuild, the Padres had the luxury of manipulating their roster to retain both.
• Hot Stove Tracker
That strategy has forced the Cards to rethink who they protect and more equally weigh future potential with proximity to the Majors. The decisions, however, to add O'Neill, Gomber and Mercado was relatively straightforward.
The Cards acquired O'Neill in a Trade Deadline swap with the Mariners in July, and the former third-round pick finished the year with 31 homers and 95 RBIs at the Triple-A level. O'Neill, 22, has a career .850 OPS in the Minors.
While the organization is saturated with outfielders, it doesn't have a plethora of prospects with the sort of natural power that O'Neill has shown. He could fill a need in a corner-outfield spot as soon as next season.

Gomber is also methodically pushing his way toward St. Louis. A fourth-round selection by the Cardinals in the 2013 MLB Draft, the lefty went 10-7 with a 3.34 ERA, 1.168 WHIP and 140 strikeouts over 143 innings at Double-A in '17.
While he has pitched exclusively as a starter in the Minors, Gomber could be an option for either the Cardinals' bullpen or rotation at some point in 2018.
Drafted as a high school shortstop in '13, Mercado was a full-time outfielder for the first time in 2017. He coupled a smooth defensive transition with his best offensive season, slashing .287/.341/.428 with a .769 OPS in 120 games with Double-A Springfield. Mercado, then followed with a standout showing in the Arizona Fall League.

Gonzalez, 22, is further away from the Majors than the aforementioned three, but the Cardinals are intrigued by his trajectory. The right-hander went 4-7 with a 4.33 ERA in 18 games (15 starts) for Class A Advanced Palm Beach last season. In 79 innings, Gonzalez struck out 72, walked 30 and posted a 1.37 WHIP.
The Cardinals signed Gonzalez as a non-drafted free agent out of Venezuela in September 2012.
The most notable player left unprotected by the Cardinals is corner infielder Patrick Wisdom, who slugged .507 with 25 doubles, 31 homers and 89 RBIs for Triple-A Memphis last season. He will be available for Rule 5 selection, along with Daniel Poncedeleon, Matt Pearce, Trey Nielsen, Andrew Morales and others. Wisdom has been passed over in the Rule 5 Draft before.