"It's obviously not what you want to hear. Hopefully it's something short," Girardi said. "The evaluation from the doctor was his strength was really good, but we've got to see him. He'll have more MRI tests tomorrow."
The 27-year-old had performed well in the Classic, hitting .348 (8-for-23) with four doubles, a home run and eight RBIs in six games to help the Dutch advance to the semifinals. He felt stiffness while throwing before Saturday's exhibition against the D-backs.
"That's always your concern when guys play in the WBC, is they're not with you," Girardi said. "They're not in your hands and that's the hard part. I mean, arm injuries can happen, that's the bottom line. The thing is, he hasn't played a ton of defense."
Gregorius batted .276 with 20 home runs and 70 RBIs in 153 games for the Yankees last season, setting career highs in hits (155), doubles (32), homers, RBIs and extra-base hits (54).
Girardi said that the best-case scenario would be that Gregorius takes some time off and is ready for Opening Day.
"I mean, we saw the strides he took last year, offensively and defensively," Girardi said. "What he did, that [would be] a big loss."
Should Gregorius not be ready for the beginning of the regular season, the Yankees' options at shortstop would include Ronald Torreyes, Tyler Wade, non-roster invitee Ruben Tejada and possibly even top prospect Gleyber Torres, who is hitting .444 (12-for-27) with two homers and seven RBIs this spring.
Yankees second baseman Starlin Castro said that he would be willing to return to shortstop if necessary.
"I can do that, for sure," Castro said. "That's my natural position. I've been doing this for a while. If they ask me, I'm going to say yes. We'll see. Hopefully he's all right."