At least 21 members of the Marlins' traveling party have been infected and the team hasn't played since Sunday in Philadelphia.
Diaz consistently tested negative while the Marlins' season was on hold, but he decided to become the first Miami player to opt out. He played in two of the Marlins' three games before their season was halted.
Diaz batted .173 as a rookie last year, but the organization has big hopes regarding his potential.
Diaz explained his decision in a story posted on his Instagram page: "This has been a tough week to see so many of my teammates come down with this virus, and see how quickly it spreads. After much deliberation and thought, I have made the difficult choice of opting out for the remainder of the 2020 season. This has been a decision that I have discussed with my family, and I feel it's the best one for me and my overall well-being. I will deeply miss my teammates and competing on the field. I wish my brothers the best and look forward to taking the field again with them soon!!"
While the coronavirus-ravaged Marlins will be without the services of Diaz, early Saturday morning they acquired left-handed reliever Richard Bleier for a player to be named in a trade with the Baltimore Orioles.
Bleier went 3-0 with a 5.37 ERA and four saves in 53 games last season. This year, he has pitched three scoreless innings in two appearances.
Although Bleier, 33, has been one of the team's most effective pitchers out of the bullpen, the rebuilding Orioles have been shuffling their roster over the past two seasons and decided to unload him.
The trade was announced after midnight early Saturday morning after Baltimore beat Tampa Bay 6-3 in a game delayed nearly an hour by rain. Bleier was not used and learned of the trade after the final out.
Bleier -- who grew up in Florida and pitched for Florida Gulf Coast University -- said he was "definitely surprised" by the deal.
"I'm glad that I have value to another team, but at the same time this was such a comfort for me here in Baltimore and I'm going to miss it," he said.
Bleier expressed no apprehension about pitching for a team that's been ravaged by COVID-19.
"Hopefully, everything is under control and I'm not going into a situation that's unsafe," he said. "But it's definitely something that I'm aware of. No, I'm not overly concerned. They're not going to bring me somewhere there's going to be a problem."
The Marlins had no new positive tests as of Saturday morning, according to ESPN and a report.
Information from The Associated Press was used in this report.