The pitch from Yu Darvish appeared to hit Alonso on the right wrist. Alonso appeared to swing, but the Mets challenged and it was ruled a hit by pitch and Alonso was awarded first base. He had headed to the clubhouse by then and was replaced by pinch runner J.D. Davis.
When the Mets went back in the field, Davis took over for Alonso at first base.
The Mets said initial X-rays came back negative on Alonso, but he'll undergo further testing to confirm that preliminary diagnosis.
New York has been hit by a pitch 40 times this season, six more than any other major league team. Alonso has been nailed seven times.
Mets outfielder Starling Marte departed with left quad tightness apparently suffered while sliding. He initially stayed in the game before exiting.
Alonso came into the game tied with Mookie Betts of the Los Angeles Dodgers for the National League lead with 16 home runs. The two-time Home Run Derby champion leads the majors with 54 RBIs and is tied for 10th in Mets history with 122 career homers.
Alonso, the 2019 NL Rookie of the Year, hit safely in his previous four games, going 6-for-18 with eight runs, a double, three homers, seven RBIs, eight runs, a walk and a 1.257 OPS.
The Associated Press contributed to this report