Nicolino will become the third lefty in Miami's rotation, joining Wei-Yin Chen and Adam Conley. In four starts at New Orleans, Nicolino has gone 2-2 with a 3.60 ERA.
Also on Tuesday, the club announced lefty Chris Narveson has been outrighted to New Orleans. Narveson was designated for assignment last week and cleared waivers.
It's been a rough three starts for Cosart, who is 0-1 with a 7.98 ERA. Throwing strikes has been an issue, as the right-hander has walked 15 in 14 2/3 innings to go with eight strikeouts.
"Command," manager Don Mattingly said. "I think he had  walks in [14 2/3] innings. He had 30 some base runners. That's not a very good recipe."
Cosart is expected to make his first start for New Orleans on Thursday.
"It's probably the right thing right now," Mattingly said. "Give him a chance to get it together. I think the walks were more the thing. Just the lack of command is really what we wanted him to try to get back in order."
In his most recent start -- an 8-1 loss at San Francisco on Friday -- Cosart gave up six runs on eight hits with six walks (two intentionally) in 4 2/3 innings.
For Cosart, the rough start to the season comes after an impressive Spring Training, when he won the final rotation spot. In Grapefruit League action, Cosart didn't allow a run in 9 2/3 innings, striking out nine while walking five. His velocity was routinely between 94-96 mph, occasionally reaching 98.
After his loss at San Francisco, Cosart said he wasn't comfortable with his mechanics. In the spring, he had his hands higher, but he was asked to lower them to his belt when the season started.
Cosart said the change didn't "feel natural," and his velocity average this season is 93.74 mph.
In his bullpen session on Sunday at San Francisco, Cosart reverted back to having his hands higher. At New Orleans, he will get a chance to iron out his delivery.
"We still have a lot of confidence in him and what he's capable of doing," Mattingly said. "Right now we felt like this was the best thing for him."
When Nicolino was optioned in Spring Training, Mattingly referred to the lefty as "our sixth guy."
The 24-year-old is not overpowering, but he impressed Mattingly with his poise, ability to throw strikes and get outs. In 25 Triple-A innings, Nicolino has 13 strikeouts and five walks and a WHIP of 1.12.
In 2015, Nicolino logged 74 big league innings, going 5-4 with a 4.01 ERA in 12 starts.
"We expect him to come up and pitch well," Mattingly said.