San Diego manager Bud Black opted to give Upton a day to rest in the series finale at Wrigley Field. The Padres open a four-game series on Monday in Denver against the Rockies.
"It's truly day to day. We're optimistic about it. We will watch it, treat it and hopefully it will be all right tomorrow," Black said. "We're not going to push this.
"When you're talking muscle strains, you never really know. We're going to treat the heck out of it. The player truly knows how he feels. Hopefully he can give us a great evaluation of how he feels."
Upton sustained the injury going first to third in the ninth inning as the Padres erased a four-run deficit to tie the game. He told Black about the pain but also said he wanted to hit in the 10th inning.
Upton doubled in the 10th inning and was replaced by pinch-runner Will Venable, who remained in the game in left field. Venable got the start in left field on Sunday.
"I didn't want to hurt it more trying to score. Obviously, it would've been the go-ahead run, so I wanted to make sure," Upton said.
Black said the smart thing to do Sunday was keep Upton from the lineup, the first time he hasn't started a game this season. He's hitting .326 in 46 at-bats with a team-leading three home runs. He is second to teammate Matt Kemp in RBIs with eight (Kemp has nine).
"He's moving around OK, but [he's] not quite where he needs to be to start a game, and all of us feel comfortable about it," Black said. "We want him in there, he wants to be in there."
Upton said he's itching to get back in the lineup and to continue his strong start offensively. That said, he wants to make sure he's completely healthy before jumping back in the lineup.