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"We've got to make a determination if he's going to fit or not," manager Terry Collins said.
Logan Verrett started Wednesday in place of Matz, with either Tim Stauffer or Sean Gilmartin set to take his place on Thursday.
"It's still lingering a little bit, so we're just going to take it day by day," Matz said. "We're not going to push it or anything."
Though this is the second time Matz has altered his schedule due to back discomfort, his last issue -- which landed him on the disabled list for over two months -- involved a left lat strain. This is on the opposite side of his body, easing the worst of his worries. Matz underwent medical tests, all of which came back negative, on Wednesday afternoon.
"When you wake up with a crick in your neck, it's kind of like that, just in my upper back," said Matz, who is 4-0 with a 2.27 ERA in six starts. "I'm not very concerned. It's going to go away. It's just a matter of when."
That is the most significant question for the Mets, who hope to gauge Matz's health by inserting him into a game this weekend -- likely a relief appearance in tandem with one of their starting pitchers.
If that goes well, Matz could still serve as the No. 4 starter in the NLDS for the Mets, who have a one-game lead on the Dodgers for home-field advantage in that series (Watch the races on MLB.TV). But the team is wary of using Matz in the NLDS, having him suffer a setback and then needing to replace him on the roster. That would make him ineligible for the NL Championship Series should the Mets advance to it, so the Mets are considering using Bartolo Colon in the NLDS rotation instead of Matz.
For now, they are waiting and watching, hopeful that Matz makes a rapid recovery.
"We've still got some time this weekend to get him some innings," Collins said. "But I just don't feel comfortable putting him out there. I know he'll go out there, but we don't need to subject him to having it get worse.
"You're talking about a young guy who's had a back problem that put him on the DL for [two months]. He comes out, throws [four] starts -- and I realize it's not the same issue, it's on the other side of his back, but it's still an issue. We've got to get this guy ready."