The Nationals will likely place rookie reliever Cole Kimball on the 15-day disabled list before tonight’s game and will summon Class AAA Syracuse reliever Ryan Mattheus to the majors. Mattheus will be available tonight in San Diego. The team has not yet made either transaction official, although Syracuse announced the Nationals have purchased Mattheus’s contract.

Kimball has allowed three earned runs in 14 innings, strikeout 11 and walking 11. Last night, Kimball entered in the sixth inning with the bases loaded and allowed two inherited runners to score, walking in a run and yielding a sac fly.

Mattheus, 27, has never appeared in the majors. Last year, he appeared in only 10 games in the Nationals’ minor league system after completing his recovery from Tommy John surgery. This year, in 24 2/3 innings with Class AA Harrisburg and Syracuse, Mattheus has a 1.46 ERA with 28 strikeouts and eight walks.

The Nationals acquired Mattheus in the 2009 trade that sent Joe Beimel to the Colorado Rockies. Before his shoulder reconstruction surgery, Mattheus became a highly regarded Rockies prospects. He has been on the Nationals’ 40-man roster since last season, and this offseason the Nationals signed him to a one-year, major league contract.

>>> Meantime, Ryan Zimmerman will not appear in a rehab game tonight, taking a schedule day off. He will continue Saturday in Class AAA Syracuse, the Nationals announced. For now, the Nationals plan on Zimmerman returning, and playing his first major league game since April 9, on Tuesday at Nationals Park.