MIAMI -- One pitch may have swung the outcome of the game on Monday night, but it didn’t change the bottom line in terms of the Marlins’ big picture.

Pablo López's return to the rotation was filled with plenty of encouraging signs, but unfortunately it was marred by a three-run home run to Freddy Galvis in the fifth inning that lifted the Reds to a 6-3 victory over the Marlins at Marlins Park.

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On the injured list since mid-June with a right shoulder strain, Lopez showed plenty of promise and very little rust.

“It's a good feeling knowing that my health is there,” said the 23-year-old Lopez, who worked five innings and logged 86 pitches. “I'm just going to keep working on my recovery, and all that process starts right now.”

Concerns over Lopez’s arm strength were quickly erased. According to Statcast, his average four-seam fastball was 94.1 mph, and it maxed out at 95.5 mph. Entering the game, Lopez had averaged 93.7 mph, slightly above the MLB average of 93.4 mph.

“I felt like my fastball was there and it was playing up in the zone,” Lopez said. “That's part of the rehab process, trying to build that endurance and knowing that your strength and velocity will be there.”

Monday was Lopez’s 15th start, and if he can hold up for the rest of the season, Miami will have more evidence as to whether he can be counted on as a rotation piece when Spring Training opens in February.

Lopez allowed four runs on five hits and one walk in five innings, with three strikeouts.

“I thought he was aggressive,” Marlins manager Don Mattingly said. “We have talked about this as a club, with all the walks last week. We wanted to show we were aggressive. Get into the strike zone and attack.”

In two of Miami’s games against Philadelphia over the weekend, the staff walked 10 batters.

Neil Walker blasted a two-run homer off Sonny Gray in the third inning, which gave Miami a one-run lead until Galvis’ opposite-field blast in the fifth. And in the ninth inning, Jorge Alfaro blistered an opposite-field homer.

Besides the two home runs, the Marlins didn’t do much off Gray, who worked six-plus innings, or the Reds’ bullpen. Miami had just five hits.

“I played with the guy last year [on the Yankees], so I knew when he was on, the type of stuff that he does,” Walker said of Gray. “That's really what he's been doing all year.”

In the fifth inning, Lopez issued a leadoff single to Nick Senzel and a walk to Josh VanMeter. After a mound visit from pitching coach Mel Stottlemyre Jr., Galvis drove Lopez’s first offering -- a 93.2 mph fastball -- over the wall in left-center.

“It wasn't a bad pitch,” Lopez said. “Maybe I could have got it a little more down, to get him to hit it on the ground. He put a good swing on it.”

Making his first start since a June 15 win against the Pirates, Lopez effectively mixed his pitches, throwing 42 four-seam fastballs, 17 changeups, 14 curveballs and 13 two-seam fastballs. His top two-seam speed was 95.1 mph.

“He's got really, really good stuff,” Walker said of Lopez. “For as young as he is, the sky’s the limit for him. It's always fun to play behind him because he attacks guys, there's a lot of balls put in play. But he also has the ability to put guys away.”