The Nationals will play with 24 players tonight and have not announced Mattheus’s replacement on the active roster. It will be a position player, though, once again giving the Nationals a standard seven-man bullpen and five-player bench. Outfielder Roger Bernadina and first baseman Chris Marrero are the likeliest candidates to join the Nationals on Saturday in Cincinnati.

Mattheus, 27, left Thursday night’s game after Johnson and pitching coach Steve McCatty noticed a lack of velocity on Mattheus’s pitches. Mattheus had also left in the middle of an appearance one week earlier, after he experienced tightness following a weight-lifting session. Late Thursday night, Mattheus felt “nothing out of the ordinary” but understood the Nationals’ caution.

“He’s had a great year,” Johnson said. “We’re in a position where we’re not fighting for a playoff spot. He’s too good of a young pitcher for us to risk it.”

Johnson factored in Mattheus’s history in deciding to place him on the disabled list. Mattheus’s last full season came in 2008, and he missed most of 2009 and most of 2010 while recovering from Tommy John surgery. Mattheus has thrown 54 2/3 innings over 54 appearances between the majors and minors this season, more than his previous two years combined. With Mattheus’s arm not used to the wear from a full season, Johnson exercised caution.

“He needs the rest,” Johnson said. “And he doesn’t have anything to prove.”

Since making his major league debut in early June, Mattheus emerged a late-inning reliever for the Nationals, compiling a 2.40 ERA in 30 innings despite more walks per nine innings (4.2) than strikeouts (3.3). He stranded 20 of the 28 base runners he inherited.

Johnson said Mattheus might not pitch again this season, “but then he may be fine and come back after a little rest and pitch.” Without Mattheus, the Nationals may have to rely more on Todd Coffey and Henry Rodriguez in the late.

“I trust Henry right now with a lead,” Johnson said, but Rodriguez has pitched erratically – at best – lately. Since the all-star break, Rodriguez has allowed 15 earned runs and four unearned runs in 19 innings. He throws the game’s hardest fastball (he’s struck out 19 over that span) but has still not shown the ability to control it (he’s also walked 17 and thrown five wild pitches). Still, Johnson remained confident in Rodriguez.

“My first year in the big leagues, I somebody wilder than him, named Doug Sisk,” Johnson said. “He got me on the Rolaids. He also had a good sinker. He could walk two or three and get out of it. Henry is a lot the same way. When he’s got everything working for him, he’s got an outstanding curveball, an outstanding fastball, a good changeup. It all just takes experience. Doug got a lot better – I was taking less Rolaids. Henry is going to do the same thing.”