Ranked by MLB Pipeline as the organization's No. 6 prospect, Sierra was in the starting lineup and went hitless in four at-bats with two strikeouts in the Marlins' 9-3 win over the Braves. He batted eighth and played center field.
"We didn't want to bring him if there wasn't going to be playing time," manager Don Mattingly said. "It will be a little bit in a sense like Lewis Brinson, who you are trying to develop as you go. But we felt like it was the right time."
Injuries created the need for an outfielder who can play center field, and with his blazing speed, Sierra can play all three outfield spots. Brinson is on the disabled list with a right hip bone bruise, while Cooper reinjured his wrist on Friday at Tampa Bay -- the same injury that kept him out three months in the first half.
Sierra is new to the Marlins' system, but he is no stranger to the big leagues. He appeared in 22 games for the Cardinals last year, hitting .317 with a .359 on-base percentage.
"It was a great experience," Sierra said through an interpreter. "I was able to learn from a lot of veterans from my time there. They gave me a lot of advice on things to do, things not to do."
Sierra's big league debut actually came against the Braves on May 7, 2017, when he went 1-for-6 at SunTrust Park.
"The one thing that stuck with me was [the Cardinals' players] told me to always stay positive," Sierra said. "No matter what is going on, to just have a positive outlook and a positive mindset."
Miami acquired Sierra from St. Louis in December in the Marcell Ozuna trade. A native of the Dominican Republic, the left-handed-hitting and throwing outfielder was in Spring Training with the Marlins, but he suffered a left hamstring strain.
Over 86 games with New Orleans, Sierra had a slash line of .260/.287/.341 with 14 stolen bases. He batted .298 in July, and the organization felt he was ready to continue to develop with the Marlins.
"Obviously, Magneuris is a really talented kid, as far as his tools -- as far as his arm, his speed," Mattingly said. "We still think he's a work in progress from the standpoint of development. Just getting more playing time, more development. But we did feel it was the right time with Cooper being down for somewhat of an extended period."
Sierra got the nod over other Minor League players, because in the eyes of the development staff, they are seeing enough overall progress from him to make the leap.
"It starts with development, because they are the ones who see him every day," Mattingly said. "Those guys are seeing him on a daily basis, day in, day out. They see all the little things that you may not notice. It's not about just the hitting. It's more of the little things, the smaller things that are part of the game."
Marlins prospect Pablo Lopez was a teammate of Sierra's at New Orleans, and the right-hander says his fellow rookie is a team player.
"It's a great opportunity," Lopez said. "Sierra is a great player and an even better person. He's always there for you. He plays hard all the time. He supports you on the field, off the field. He's just a great guy overall."
Sierra's opportunity came because of Cooper's second injury to the same wrist. On Monday, he had an MRI and was evaluated by a specialist.
"There's some damage to the wrist and to the tendons," Cooper said. "There's inflammation in there."
Cooper was injured in the late innings Friday, landing on his wrist while fielding a sharply struck ball off the bat of Adeiny Hechavarria. He stayed in the game, but he was not available on Saturday or Sunday.
"It's just frustrating, and it's been frustrating all year," said Cooper, who will get another MRI once the inflammation subsides. "It's not what you work for all offseason -- to come in and have the same thing happen again."
All-Star catcher J.T. Realmuto has been named the Marlins "Heart & Hustle Award" winner by the Major League Baseball Players Alumni Association. The honor is presented to an active player on each of the 30 MLB teams who demonstrate a passion for the game.