Manager Bob Melvin disclosed the injury after the series finale in San Diego but said he thinks Soto should be ready when the Padres open a key series against the Los Angeles Dodgers on Friday.
Soto was hit in the shoulder by a 91 mph fastball from Arizona pitcher Tommy Henry in the bottom of the third inning. He stayed down in a crouch in the batter's box for about a minute before walking slowly to first base. He played defense in the fourth inning before being replaced in the top of the fifth.
Soto said afterward that he felt stiff when trying to swing but said he will "definitely" be ready to play in the opener against the Dodgers.
"I think he is going to be OK, hopefully after an off-day,'' Melvin said. "Initially when you get hit, kind of the strength goes away. He tried to go in the cage and swing a little bit and it wasn't great. But these things happen during the course of a season. Hopefully after a full day's rest he will be able to go the next day."
Soto was 0-for-1 with a strikeout in the game and continues to struggle for San Diego, which acquired Soto in a blockbuster trade with the Washington Nationals last month.
Entering Thursday, he had just two hits over his last eight games, tied for his fewest in such a span this season. Overall, he's hitting .232 with three home runs and six RBIs in 29 games since San Diego acquired him.
Information from The Associated Press was used in this report.