Tony Sipp did not allow a run in his last nine appearances before going to the injured list with an oblique strain. (Toni L. Sandys/The Washington Post)

NEW YORK — The Washington Nationals’ bullpen continued to shuffle Monday as the team activated left-handed reliever Tony Sipp off the 10-day injured list, designated lefty Dan Jennings for assignment and claimed right-hander Javy Guerra on outright waivers from the Toronto Blue Jays.

Sipp returns from an oblique strain in time to face the New York Mets at 7:10 p.m. at Citi Field, the first of a four-game series between two struggling National League East foes. The 35-year-old has not allowed a run in his last nine appearances, only two of which lasted a full inning, and rejoins a bullpen that has a league-worst ERA at 6.60.

“I think these last 10, 12 days were kind of good for him,” Manager Dave Martinez said of Sipp. “That he was able to take a little breather because he didn’t have spring training. So he feels really good. He had an outing the other day [and] was up to 93. So he says his arm feels good, and he’s ready to go.”

Jennings signed with the Nationals on April 15, debuted April 30 and has since made seven more appearances that led to a 13.50 ERA. It is fair to assume he will not pitch again for the Nationals, who are taking a flier on Guerra after he was placed on waivers by the Blue Jays.

Jennings allowed 16 hitters to reach base in 4⅔ innings for Washington. The 32-year-old has three days to be claimed off waivers. If he isn’t, the Nationals will have seven days to trade him. After that, once 10 days are up, Jennings could either accept a minor league assignment or elect to become a free agent.

His designation opens up a 25-man roster spot for Sipp, who was signed by Washington in mid-March to be a matchup lefty specialist. Jennings’s departure also could create the 40-man roster spot needed to add Guerra, and another move will be needed once Guerra joins the Nationals. Martinez hopes that will be as soon as Tuesday. A logical option would be to send Kyle McGowin back to Class AAA Fresno.

“He’s got four pitches. He’s been doing well,” Martinez said of Guerra. “His last outing he threw three innings, three scoreless innings. He’s been in this division, he knows the hitters, so just getting him in here and getting him settled in a little bit, I think he’s a guy that gives us a little bit more length, as well.”

Guerra, 33, is a nine-year veteran and has a career 3.44 ERA with five teams. The righty was serviceable for Toronto this season — with a 3.86 ERA in 14 innings — and was mostly a long reliever with four multi-inning appearances. He was designated for assignment in mid-April, had his contract selected by the Blue Jays on May 10 and was again designated for assignment Saturday amid a roster shake-up.

That landed him with the Nationals, a team searching for trustworthy relievers ahead of a critical stretch against struggling teams. First comes four games against the Mets in Queens. Then the Nationals host the Miami Marlins, baseball’s worst club, for four games over the weekend.

Bullpen issues could keep Washington from capitalizing on the favorable schedule. So they hope that Sipp’s return — and the possible addition of Guerra — provides even a bit of help.

LINEUPS

Nationals (19-27)

Trea Turner, SS

Adam Eaton, RF

Anthony Rendon, 3B

Juan Soto, LF

Howie Kendrick, 2B

Gerardo Parra, 1B

Victor Robles, CF

Yan Gomes, C

Patrick Corbin, P

Mets (20-25)

Jeff McNeil, 2B

Amed Rosario, SS

J.D. Davis, LF

Pete Alonso, 1B

Wilson Ramos, C

Todd Frazier, 3B

Carlos Gomez, RF

Juan Lagares, CF

Wilmer Font, P

Read more on the Washington Nationals:

Analysis: Luck, health and strength of schedule will decide Nationals’ fate

Nationals-Mets series preview: Erick Fedde’s rotation audition, Juan Soto sparks

Sean Doolittle and Joe Maddon agree: MLB should provide more clarification of toe-tap rule

Sean Doolittle on Joe Maddon’s protest: ‘Sometimes he has to remind people how smart he is’

Nationals put Justin Miller back on injured list and Tanner Rainey joins the bullpen

Nationals rookie Victor Robles is surprising people with his power