"It's something I've been dealing with for a while," Alonso said. "I really can't tell you [how much], but it's been affecting me and I kept playing with it."
On Friday, he was wearing a small protective cast on his hand, which he also injured last year. He estimated he likely wouldn't resume hitting in the cage for at least a week.
"Just for right now, just to calm down I want to rest it as much as possible so I can come back strong," Alonso said.
Alonso is hitting .210 with five home runs, 15 doubles and 22 RBIs in 229 at-bats this season. While he's out, newly called up Jake Goebbert and Tommy Medica are expected to split time at first base.
San Diego manager Bud Black said the lefty-hitting Goebbert will make his big league starting debut on Saturday or Sunday against the Dodgers.
Corey Brock is a reporter for MLB.com. Keep track of @FollowThePadres on Twitter.Will Laws is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.