The Royals traded for Gustave, who has not pitched higher than low Class A, from the Red Sox after Boston selected him in December's Rule 5 Draft from the Astros. This means if the Padres don't keep Gustave on their 25-man roster for the entire season, he would then be offered back to his original team, the Astros.
Gustave, a power arm, had a 3.68 ERA in 7 1/3 innings this spring for the Royals with six strikeouts and three walks.
The likely scenario is the Padres will use the next week to evaluate Gustave in workouts and games to see if he's a likely fit for their bullpen, one that's already brimming with internal candidates.
"It's a powerful arm. He comes with some raw stuff," Padres manager Bud Black said. "We'll have a look to see what we've got and we'll go from there. I don't think there's any guarantees with Mr. Gustave."
In his first five seasons in the Astros' system, Gustave posted a 9-18 record and a 5.73 ERA, but he had 199 strikeouts in 196 1/3 innings.
The Padres play their final game in Arizona on April 2 before heading to San Diego, where they'll play an exhibition game against the Mexico City Red Devils on April 4 at Petco Park in advance of Opening Day on April 6 at Dodger Stadium.
In order to make room for Gustave on the 40-man roster, the team placed catcher Tim Federowicz on the 60-day disabled list. Federowicz had surgery on March 13 to repair the meniscus in his right knee, and he is expected to miss three to six months.
Corey Brock is a reporter for MLB.com. Keep track of @FollowThePadres on Twitter. This story was not subject to the approval of Major League Baseball or its clubs.