ST. LOUIS -- Another injury has caused a ripple in the Mets' rotation, which will be without Steven Matz for at least the next week. The Mets on Monday placed Matz on the disabled list with what they termed "left shoulder tightness," adding that Matz has suffered "no structural damage" in his pitching shoulder.
The DL assignment is retroactive to Aug. 15, making Matz eligible to return before the end of this month -- potentially after missing just one start. The Mets did not comment beyond revealing Matz's updated diagnosis.
In a separate move, the Mets also placed second baseman Neil Walker on the paternity list. The club recalled pitcher Robert Gsellman from Triple-A Las Vegas to replace Matz on the roster, and infielder T.J. Rivera will replace Walker.
Gsellman, who could make his big league debut, will be available out of the bullpen Tuesday in St. Louis. Mets manager Terry Collins indicated over the weekend that right-hander Seth Lugo will replace Matz in the rotation.

The updated diagnosis offers some clarity for Matz, who skipped his last start in San Francisco due to left shoulder irritation. Matz stayed with the team through the weekend before taking a redeye flight back to New York City, where he underwent an MRI on Monday morning.
It has been a season full of similar examinations for Matz, a regular patient at Manhattan's Hospital For Special Surgery. In May, Matz missed a start due to what the Mets called a "flare up" in his elbow; a month later, the team revealed that a bone spur was causing ongoing issues in that joint. Matz, who underwent Tommy John surgery in 2010, is tentatively scheduled for a postseason operation to remove his spur.
Though he has mostly managed to pitch through the injury, Matz has done so without consistency. The left-hander went 2-7 with a 4.21 ERA over his last 13 starts after opening the season 7-1 with a 2.28 mark. But Matz's last outing was perhaps the best of his career: he held the Padres to one hit in seven innings, striking out eight.

His absence leaves the Mets, who have already lost fellow starters Matt Harvey and Zack Wheeler for the season, scrambling to keep five healthy arms in their rotation. Lugo made his first career start in place of Matz on Friday, holding the Giants to three runs over 6 2/3 innings.