Hernandez left camp in Port St. Lucie on Thursday morning after experiencing pain in his neck and upper back twice in the previous four days after throwing. The club said Hernandez received a cortisone shot in the area and that he is expected to resume working out once the medication has taken effect after three or four days.
Hernandez was expected to return to camp Friday.
General manager Omar Minaya said the cortisone shot had the desired effect.
"He immediately felt much better," Minaya said.
The diagnosis, Minaya said, "is basically good news," in view of what the problem might have been. He said doctors had ruled out any vertebrae problem in the veteran pitcher, who is listed as 37-years-old in the Mets' publications, but who is widely suspected to be four years older.
Minaya dismissed the notion of age being a factor, saying arthritis "can happen to anybody." He also indicated it can be treated to some degree with massage, and the GM reiterated how quickly Hernandez reacted to the cortisone.
Although the diagnosis wasn't distressing, the very fact that "El Duque" had to leave camp and will miss time plays on the concerns -- mostly spoken in private -- the Mets have about their starting pitching.
Their rotation lacks definition and proven depth even with Hernandez. The arthritis is the kind of development former Mets manager George Bamberger once likened to "stubbing the toe with an in-grown nail," i.e., something that wasn't good to begin with becoming worse.
Before the diagnosis, Minaya said he wasn't concerned about the first hiccup of the club's spring camp, and manager Willie Randolph said "he probably won't miss a beat." Hernandez is an important element in the Mets' plans for the first months of the season. He and Tom Glavine, who is all but certain to be the team's Opening Day starter, have been characterized by Minaya and Randolph as the only rotation certainties, although John Maine warrants a "probable" designation.
It was premature Thursday to assess the Mets without Hernandez. But starting pitching is undeniably the one area in which the Mets are not well-stocked.
Minaya acknowledged Hernandez experienced discomfort in his neck last season at some point after his move from the Diamondbacks to the Mets in late May. But the GM also said El Duque missed no time because of the discomfort, and that the time Hernandez did miss -- he didn't pitch from Aug. 20-Sept. 3 -- was not related to his neck.
Minaya said the discomfort began "two or three days ago" after Hernandez had thrown, and that the righty threw again before the decision was made to have his neck examined. Three veteran members of the pitching staff said they were unaware of any problem with Hernandez and that they didn't believe he had cut short his throwing.
Hernandez made 29 starts last season, 20 with the Mets. He was to have started the first game in their National League Division Series against the Dodgers, but he tore a muscle in his right calf and never pitched in the postseason.
Hernandez said earlier this week he felt fine, but he was hardly one to speak candidly about his physical condition.
This development came two days after Phillies shortstop Jimmy Rollins, handicapping the race for the National League East championship, said "Their staff is a little older," when discussing the Mets.
The Mets have Glavine, who turns 41 next month, and Hernandez. Most of the other candidates are younger than 30.
Marty Noble is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.