OAKLAND -- The A's on Wednesday added Raul Alcantara to their 40-man roster to protect the right-hander from next month's Rule 5 Draft.

The Dominican Republic native, acquired with Josh Reddick from Boston in the 2011 Andrew Bailey trade, was the only addition to Oakland's 40-man roster, which currently consists of 39 players. That means one spot remains open in advance of the Rule 5 Draft, to be held Dec. 12 at the Winter Meetings in Orlando, Fla.

The move also ensures Alcantara, who turns 21 in December, of his first invite to big league camp.

Alcantara combined to go 12-6 with a 3.11 ERA in 27 starts with Class A affiliates Beloit and Stockton. In 156 1/3 innings, the righty struck out 124 and walked just 24, for an average of 1.38 per nine innings.

He was originally signed by the Red Sox as a non-drafted free agent in 2009 at age 16. Players signed at age 18 or younger must be added to 40-man rosters within five seasons or they become eligible to be drafted by other organizations through the Rule 5 process.

Players signed at 19 years or older have to be protected within four seasons. Clubs pay $50,000 to select a player in the Major League phase of the Rule 5 Draft. If that player doesn't stay on the 25-man roster for the full season, he must be offered back to his former team for $25,000.

In other words, an international player or high school draftee signed in 2009, assuming they were 18 or under as of June 5 of that year, must be protected. A college player taken in the 2010 Draft is in the same boat.

By adding Alcantara, the A's have chosen to leave all but two of their collegiate picks from the 2010 Draft unprotected, with A.J. Griffin and Michael Choice already on the 40-man roster.

That includes right-hander Blake Hassebrock, an eighth-round pick who has a 4.40 ERA through four Minor League seasons, along with relievers Jeff Urlaub -- he struck out 40 in 46 2/3 innings following his promotion to Double-A Midland this year -- and Seth Frankoff, who posted a 2.78 ERA and 1.08 WHIP at Stockton.

First baseman Anthony Aliotti is also eligible for the Rule 5 Draft for a second straight year. He hit .350 with 12 home runs and 51 RBIs in 91 games for Midland, before advancing to Triple-A Sacramento, where he batted .266 with a pair of homers and 20 RBIs in 42 games.

Leaving these players exposed to the Rule 5 Draft isn't a major risk, since clubs are forced to keep Rule 5 players on its 25-man roster for an entire season or offer them back to their former team.

As has been the case for several years, the A's didn't lose a player in the Major League portion of the Rule 5 Draft in 2012. They also didn't take any players, though they ultimately reeled in Nate Freiman on a waiver claim from Houston, just months after the first baseman was plucked by the Astros in the Rule 5 Draft from the Padres.