NEW YORK — Mets star pitcher Noah Syndergaard will be sidelined through at least the All-Star break because of a torn muscle behind his right arm.

Syndergaard, the hardest-throwing starting pitcher in the major leagues with an average fastball velocity of 98.7 mph this year, partially tore his latissimus dorsi against Washington last Sunday. The 24-year-old right-hander has been told not to throw for about six weeks and he will need a long period to build up arm strength again, equivalent to a full spring training.

“It stings,” Syndergaard said Saturday, two days after he was examined in Los Angeles by Dr. Neal ElAttrache.

Syndergaard was scratched from an April 27 start against Atlanta with what the Mets said was biceps and shoulder discomfort. Syndergaard pitched April 30 at Washington, lasted just 11/3 innings and went on the 10-day disabled list the following day.

Mets General Manager Sandy Alderson wouldn’t give a timetable for Syndergaard’s return and said the pitcher could be put on the 60-day DL.

“Realistically, it’s going to take a while. We don’t know when he’s going to be back,” Alderson said. “It’s going to be much later in the season.”

ORIOLES: Closer Zach Britton is back on the disabled list with a left forearm strain – the same injury that recently sidelined him for two weeks.

Britton came off the 10-day disabled list Tuesday and appeared in two games before feeling discomfort in his pitching arm. He had an MRI on Friday and was placed on the 10-day DL on Saturday.

“The MRI was about the same as the last time. Maybe a little bit more inflammation,” Britton said. “The elbow looks good, so this is just a muscle issue.”

The left-hander acknowledged that he probably came back too soon.

“The doctors and the trainers wanted me to be a little more cautious with it at the time, maybe take another week,” Britton said. “I was kind of over sitting on the bench watching games and felt I was in a good enough position to come back. Obviously, I wasn’t.”

Britton is 5 for 5 in save opportunities with a 1.00 ERA.

He intends to visit Dr. Neal ElAttrache, an orthopedist based in Los Angeles, on Monday.

Brad Brach will serve as the closer in Britton’s absence. He’s converted six of seven save opportunities this season.

• The Orioles also put right-hander Gabriel Ynoa on the 10-day DL with a strained hamstring.

GIANTS: San Francisco promoted outfielder Justin Ruggiano from Triple-A Sacramento and designated outfielder Drew Stubbs for assignment before its game against Cincinnati.

PIRATES: Right-hander Jameson Taillon was placed on the 10-day disabled list with groin discomfort.