Murphy departed Thursday's Grapefruit League game against the St. Louis Cardinals after singling and scoring on a sacrifice fly in the top of the first inning. The team initially labeled the injury "tightness." An MRI taken Friday revealed the more extensive issue.
"He'll be out a week or so, maybe a little longer," Alderson said. "Hamstrings take longer than people want to admit."
With the Mets slated to open the season at Washington in 16 days, Alderson acknowledged Murphy could be facing a season-opening DL stint. Prospects Matt Reynolds and Danny Muno -- neither projected to make the team at the start of camp -- would be the primary candidates to start if Murphy is unavailable, the GM added.
It has been a rough week for the Mets from an injury perspective.
Right-hander Zack Wheeler received word Monday that he requires Tommy John surgery. Left-handed reliever Josh Edgin underwent that same elbow procedure Tuesday. And fellow reliever Vic Black has been slowed by what is being labeled shoulder "weakness" -- although manager Terry Collins suggested Black would resume throwing Saturday on flat ground, now that he has completed anti-inflammatory medication.
Collins has suggested Black could be DL-bound to open the season, too.
With Murphy eligible for free agency next offseason, prospect Dilson Herrera has been portrayed as the heir apparent at second base. However, Herrera already has been optioned to minor league camp. And Alderson said "right now" there are no plans to bring back Herrera -- either now or at the start of the regular season. Herrera is slated to open the season at Triple-A Las Vegas.
Reynolds and Muno have yet to make their major league debuts. Reynolds hit a combined .343 with six homers and 61 RBIs and had a .405 on-base percentage in 543 plate appearances between Double-A Binghamton and Vegas last season. He is a former second-round pick out of the University of Arkansas.
Muno, a former eighth-round pick out of Fresno State, hit .259 with 14 homers and 62 RBIs and had a .372 on-base percentage in 435 Triple-A plate appearances last season.
Alderson cited an MLB rule in explaining in part why he preferred Reynolds and Muno over Herrera. Neither Reynolds nor Muno is on the 40-man roster. So if either became injured during the remainder of camp, he would not have the right to go on the major league DL and accrue service time to open the regular season. If Herrera returned and became injured in camp, he could be entitled to the major league DL.
Backup middle infielder Ruben Tejada went unmentioned as a starting candidate.
Alderson added that Wilmer Flores will remain the shortstop no matter what. Asked if the Mets might consider Flores at second base and Reynolds or someone else at shortstop if Murphy missed time, the GM said it had taken this long to convince the public that Flores is capable of playing shortstop, so he is not budging from the position.
"Wilmer is the shortstop. Wilmer is the shortstop," Alderson said twice. "It's taken us this long to convince you guys that Wilmer is the shortstop."