Matt Grace at Syracuse. (Herm Card/Courtesy of the Syracuse Chiefs)

Ahead of Thursday’s deadline to protect players from December’s Rule 5 draft, the Nationals added four prospects — right-handed starter A.J. Cole, outfielder Brian Goodwin, infielder Wilmer Difo and left-handed reliever Matt Grace — to the 40-man roster.

The Nationals already had two open roster spots. And in order to clear another two spots, the Nationals placed reliever Ryan Mattheus on outright assignment and he elected free agency. Mattheus was a key part of the Nationals bullpen in 2012 but struggled ever since he broke his hand in 2013 when he punched a locker and spent most of 2014 in the minors. Infielder Pedro Florimon, who the Nationals claimed off waivers from the Twins late last season, was claimed off waivers by the Pirates.

The Rule 5 draft takes place Dec. 11 in San Diego, the final day of the winter meetings. Any player selected in the draft must remain on the 40-man roster the entire season — if not, he is returned to his original team. The Nationals’ 40-man roster is now full, so if the Nationals make an offseason move, it would require more room.

Cole, 22, was the most obvious addition to the 40-man roster. He is the Nationals’ No. 2 prospect, after starter Lucas Giolito. Cole, who has standout control of his mid-to-high 90s fastball, went 13-3 and 3.16 ERA this year, his best professional season to date, and finished at Class AAA Syracuse. He is expected to be in big league spring training, and could serve as rotation and bullpen depth during the season.

Grace, 25, also had his finest season to date, posting a 1.17 ERA between Class AA Harrisburg and Syracuse this season. He is durable, tough on left-handers, throws heavy sinkers, pitched well in the Arizona Fall League and could be a cheaper option for the Nationals’ bullpen next season. Consider him a darkhorse bullpen candidate in spring training.

Difo, 22, could have been left unprotected because he spent the season at Class A Hagerstown but the Nationals didn’t want to risk losing the talented middle infield prospect, one of the few in the system, and rewarded him for a breakout season. The Dominican native hit .315/.360/.470 with 14 home runs and stole 49 bases this season, while playing both shortstop and second base. He was the MVP of the South Atlantic League and, based on a generous estimate, could be pushing toward the major leagues in 2016.

Goodwin, 24, had a down 2014 season but still carries plenty of potential. He was surpassed by outfield prospects Michael A. Taylor and Steven Souza Jr. on the outfield depth chart. He hit .219/.342/.328 with four home runs, six stolen bases and 95 strikeouts in 81 games last season at Syracuse. He was the 34th overall pick in the 2011 draft and the Nationals handed him a $3 million signing bonus, so they protected their investment by adding him to the 40-man roster.

The most notable omission was power-hitting corner infielder Matt Skole, but it makes sense. The left-handed hitter is intriguing because of his power but he was injured in 2013 and struggled during his return last season. He hit .241/.352/.399 with 14 home runs and 127 strikeouts in 132 games at Harrisburg. A team willing to carry him on the 40-man roster all season could snatch him up.

Two kinds of players can be taken in the Rule 5 draft: those who have played four seasons and debuted as a professional after they turned 19, and those who were 18 at their debut and have played five seasons. For this year, essentially, that means players drafted out of college in 2011 and players picked from high school in 2010. Teams can prevent opposing clubs from taking its players by adding them to their 40-man roster.