NEW YORK -- New York Mets left-hander Steven Matz's planned return from the disabled list on Thursday is off.
Matz experienced renewed shoulder tightness while throwing off a mound on Sunday and again while throwing on flat ground on Monday and the Mets temporarily have shut him down.
Matz said he is dealing with a shoulder impingement that is causing rotator-cuff irritation. He predicted he will pitch again this season. Matz added that surgery will not be required to remedy the issue. It's more about correcting posture.
"Just a little irritation -- it's still bugging me a little bit," Matz said Tuesday afternoon. "I don't quite feel like I can let it go yet. ... The doctor said everything structurally looks OK. It's just a classic impingement, I guess, is what you can call it.
"I felt like I was making progress, and then I threw off the mound a little bit, and I felt OK coming out of the there. And then the next day, which was yesterday, I tried to throw and it was kind of barking at me a little bit again. It didn't cause me to stop throwing. I was able to get my tossing in. But for me to get on the mound and throw a bullpen and tell them I'm ready for a game would just be unrealistic in my mind."
Matz already is planning to undergo surgery after the season to remove a bone spur from his pitching elbow.
The Mets have expressed confidence that Matz's elbow issue has not resulted in him altering his mechanics and caused the shoulder problem. Still, Matz said about that topic: "There's really no way of telling. My arm was really feeling great, which is why this is a big disappointment. A few days after that last game I threw, something just started barking at me. It kind of really hasn't subsided since."
The Mets had planned to bump Robert Gsellman from the rotation with Matz's expected return on Thursday, but now the rookie right-hander will remain in the rotation.
Jacob deGrom, skipped Monday to provide extra rest after recent struggles, now will reenter the rotation for Thursday's series finale against the Miami Marlins. Noah Syndergaard, Gsellman and Seth Lugo then will face the Washington Nationals this weekend at Citi Field.
The Matz setback was not the only bad news revealed by manager Terry Collins on Tuesday. While cautioning that he was not authorized to provide specifics, Collins said the lower-back issue that is troubling second baseman Neil Walker is expected to remain a problem for the remainder of the season. As a result, Walker's play down the stretch may not be consistent. Walker is out of the lineup for a third straight game on Tuesday.
Homecoming: Collins confirmed that outfielder Michael Conforto will be among the first wave of call-ups when rosters expand on Thursday. Conforto is hitting .415 with eight homers and 26 RBIs in 123 at-bats with Triple-A Las Vegas in 2016. He hit .218 with 11 homers and 33 RBIs in 266 at-bats in the majors this season. ... The Mets extended their player-development agreement with Las Vegas through the 2018 season. The location is not optimal because the distance from New York requires promoted players to take redeye flights in order to join the big-league club.