The Nationals summoned Ross Detwiler in need of a starter tonight after Saturday’s doubleheader, and if Davey Johnson has his way Detwiler won’t leave anytime soon. Johnson has wanted both a pitcher who can swing between the rotation and the bullpen and a second lefty reliever, and Detwiler could fill both roles.

Johnson envisions using Detwiler as “insurance,” a pitcher capable of rescuing a starter who is either scratched unexpectedly or is knocked from the game after a few outs, like Jason Marquis on Sunday.

Detwiler would also become useful as a sixth starting pitcher as the Nationals decrease Jordan Zimmermann’s workload. Zimmermann, who has become the Nationals’ clear ace, is working on a 160-inning limit in his first full season after Tommy John surgery. He’s already thrown 108 2/3 innings in 17 starts, and at that rate he’ll make only eight or so starts this year.

In order to extend his season, the Nationals will look to skip Zimmermann when off days arise, Johnson said. Johnson met today with pitching coach Steve McCatty today to discuss whether Zimmermann will start again before the all-star break, and they will make a final decision tomorrow. Zimmermann, who started Monday, would make his next start Saturday.

Johnson, though, can also see a scenario in which Zimmermann and Detwiler are on the same staff. “I will still use [Detwiler] in the rotation, and out of the ‘pen as if he was starting,” Johnson said. “He won’t be coming back on short rest. He’ll get his side work. You guys know my feelings, anyway. I’d like to have another left-hander. I’d like to use him as a starter out there.”

For some clarification on Collin Balester’s DFA: The Nationals had to designate him because he made his major league debut more than three calendar years ago. Balester will spend the next 48 hours in a kind of purgatory on optional waivers, and on Thursday he’ll still be on the 40-man roster and pitching for Syracuse. Effectively, it’s the same as an option, but it’s a different procedure necessitated by the date he made his big league debut.