Trailing 5-0, Black lifted starting pitcher Eric Stults to take a stab at cutting into the Marlins' lead.
"He's got some faith in me, which is great to see," Medica said.
Medica got some pitches to hit, but fouled off Koehler offerings on a 2-0 and then a 3-1 count. He ended up popping up in foul territory for the first out.
"I got into good counts, 2-0 and 3-1, where I knew he wasn't going to mess around and try to walk me," Medica said. "He came right after me and I took some good hacks. You want to get something to the outfield and at least get one run.
"When you get in the power counts like that, and he's going to give you something to hit, you've got to take advantage of those."
To that point, Black hadn't used anyone on his five-man bench. But he opted for Medica in that spot for a few reasons.
"He had a very good Spring Training and I thought his at-bats, as limited as they've been this year, have been pretty good, with good swings," Black said. "In a 2-0 count, he had a good swing, and at 3-1, he just missed it.
"I think he's seeing the ball well. I think that matchup was our best one for Tommy to get a hold of one. This is baptism under fire for Tommy. He will get his starts moving forward, but part of the reasons he's on this team is for situations like that."
Medica, who has one hit in his first six at-bats this season, got the start Sunday in left field against the Marlins.
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.