MIAMI -- When the Marlins sent Braxton Garrett out of big league camp in Spring Training, manager Don Mattingly saw the rookie left-hander’s disappointment. It was then that Miami’s skipper knew the organization’s No. 21 prospect was confident in his ability to succeed in the Majors.

In his season debut on Sunday afternoon, Garrett made just one mistake -- a hanging slider -- that Donovan Walton belted for a fourth-inning grand slam in the Marlins' 5-1 loss to the Giants at loanDepot park. With the defeat, Miami settled for a four-game split with San Francisco, as the club has been unable to win consecutive games since May 16-17.

“I thought today was a good indication of [a guy being ready],” Mattingly said. “Listening to him in his pregame meeting, he kind of knew what he wanted to do. [Catcher Jacob Stallings] had a good idea of how to use his stuff. We'll see, but he seems more prepared now than ever to be here and have success.”

Garrett held the Giants scoreless through three frames, stranding a pair of runners in each of the first two innings. Two of the batters reached on two-strike hit-by-pitches, while the other two hit soft-contact singles. Garrett worked a perfect third before encountering trouble in the fourth.

Thairo Estrada and Jason Vosler knocked back-to-back one-out singles and Curt Casali walked to load the bases. Following a mound visit, Walton jumped on Garrett’s slider -- his 72nd pitch of the day -- and sent it over the right-field wall. That was it for Garrett, who had built up to 68 pitches in his most recent start at Triple-A Jacksonville after being shut down for a few days with minor inflammation in the back of his left shoulder.

“He was doing a lot of strikes early,” Giants manager Gabe Kapler said, “and I thought we were having good quality at-bats and lengthening those at-bats. We knew that he was probably on some sort of pitch count -- like somewhere in the 75-80 range -- so we wanted to kind of push that pitch count up. But he did a nice job of attacking the strike zone. Outside of that one pitch, I thought he was good. And even that pitch, I'm not criticizing it, it's just baseball.”

Garrett translated his penchant for strike-throwing in the Minors to the Majors. In his previous 10 MLB outings (nine starts and one relief appearance), the 24-year-old southpaw recorded an average of 5.4 walks per nine innings -- compared to just 3.3 BB/9 in the Minors.

In Sunday's series finale, Garrett recorded first-pitch strikes to 15 of 18 batters (83%) -- his best mark in his 10 career MLB starts. His strike percentage (69.4%) ranked second. Location is crucial for Garrett given his arsenal: His fastball usually ranges from 88-91 mph. On Sunday, it averaged 91.3 mph.

Over the extended offseason, Garrett fixed his posture and lowered his arm slot, seeing a modest uptick in velocity as a result. He also changed his slider grip so that it has more bite and depth. Though it’s his newest pitch, he threw it the most in Sunday’s outing.

“A guy like me has to throw strikes,” said Garrett, who walked one and struck out five over 3 1/3 innings. “I don't have the 99 [mph], the 98 [mph] like a few of our guys have that can kind of get away with it here and there. Especially my first several starts before today, I've always kind of been a tad erratic. Last year, I had some mechanical things.”

The seventh overall pick in the 2016 MLB Draft, Garrett has spent parts of the past two seasons shuttling back and forth between the Minors and Majors, logging a 4.89 ERA in his MLB starts. That is by no means an easy task.

With two-fifths of Miami’s Opening Day rotation not on the active roster (Jesús Luzardo due to injury and Elieser Hernandez due to a demotion), an extended opportunity is there for the taking for rookies Garrett and Edward Cabrera (MLB Pipeline's No. 47 overall prospect).

Outside of Sandy Alcantara and Pablo López, the Marlins’ rotation has been full of question marks through the first two months of the season. Alcantara (1.81 ERA) and López (2.18) rank second and third in the National League, respectively, in ERA. The club has gone 13-9 in their starts. But Miami’s other starting pitchers have a 5.35 combined ERA, which would be the second-highest mark in the Majors. The Marlins are 9-21 in those games.

“The nerves weren't there as much as they have been in the past,” Garrett said. “My preparation’s a lot better with video and heat maps. I've learned a lot -- what to look for and things like that. I think my preparation is better, which definitely takes nerves away when you have a plan.”