Cosart, who will step on the Marlins Park mound for the first time since July 25, will face Miami right-hander David Phelps (7-6, 2.28 ERA). Cosart is 0-1 with a 4.54 ERA.
"I'm just trying to prepare like it's another major league start," Cosart said. "I get nervous before every major league start. I'm sure I will have that same nervous excitement."
Cosart said he is trying to "build off what I've been doing the past three starts," when he posted a 3.78 ERA and walked just four batters in 16 2/3 innings.
It's been a battle for Cosart since his breakthrough year of 2014. He split his time that year between the Houston Astros and the Marlins, going 13-11 with a 3.69 ERA. However, he is just 2-6 since then.
Phelps, meanwhile, has been outstanding all year, first as a set-up reliever and more recently as a starter after Miami suffered rotation injuries to Wei-Yin Chen, Colin Rea and Adam Conley. Rea, who was part of the seven-player trade that brought Cosart to San Diego, was returned to the Padres after he got hurt in his first start.
With all that adversity, Phelps has come through, posting a 1.31 ERA in his first four starts. He has allowed hitters just a .192 batting average and a .550 OPS.
The Marlins (66-61) need to win this series against a Padres team that is not in playoff contention with a lowly 53-74 record.
Miami may look to start newly acquired outfielder Jeff Francoeur this weekend.
Francoeur, who was acquired in a trade with the Atlanta Braves on Wednesday, arrived in Miami on Thursday and got an infield single as a pinch-hitter in his Marlins debut.
Among his new teammates is Martin Prado, who played with him when they were with the Atlanta Braves.
"I've played against the Marlins forever," Francoeur said. "I do know some familiar faces. I have respect for a lot of these guys and (manager Don) Mattingly and his staff.
"I'm excited. (The trade to a playoff contender) was something I needed. It was a tough year in Atlanta with all the rebuilding. I'm excited to come here and get at-bats that mean something."
In other story lines:
--Both the Padres and Marlins were named on Thursday as two of the hosts of the 2017 World Baseball Classic. Marlins Park will get first-round games March 9-13. San Diego's Petco Park will get second-round games March 14-19.
--San Diego center fielder and leadoff man Travis Jankowski has reached base in 23 consecutive games. Hitting .257, Jankowski had two hits and scored one run in his most recent game, a 6-3 loss to the Chicago Cubs on Wednesday.
--Marlins right fielder Ichiro Suzuki went 0-for-4 on Thursday and needs one more hit to tie Wade Boggs (3,010 hits) for 27th place on the MLB career hit list.