"Probably the third or fourth step after touching first, I just felt it grab," Castro said. "My upper quad locked up pretty tight and [it's] pretty sore right now. Got some treatment on it. We're kind of going to see how it feels tomorrow, how it responds overnight."
Castro is listed as day to day, and manager A.J. Hinch said the club would know more on Saturday. However, Hinch said Castro won't be in the lineup for the second game of the series at Target Field.
"Obviously best case scenario, I won't have to go on the DL," Castro said. "With the way it kind of graded out in the evaluation, we'll see day-to-day how it feels, but I'm pretty optimistic right now that it shouldn't be something that's too bad."
Castro, who is hitting .220 with 11 home runs and 30 RBIs in 90 games this season, was just starting to heat up at the plate. After going 1-for-2 on Friday, Castro now has hits in four straight games, including a walk-off home run in Sunday's victory over the Dodgers.
"I was feeling really good in the box and catching well, and just feeling good overall," Castro said. "Not the most opportune time for something like this to happen. That's probably the most frustrating part was just kind of the timing of it."
Hank Conger came on to pinch-run for Castro. In bottom of the fifth, Conger made a nice play by bare-handing a popup on Byron Buxton's bunt attempt and doubling off Shane Robinson at first base. Conger finished 0-for-2 at the plate.
Betsy Helfand is an associate reporter for MLB.com.