Robertson (6-8) allowed eight runs -- seven earned -- in five innings against the Rockies, raising his ERA to 5.47 in 18 starts. He's just 1-3 with an 8.53 ERA in his last five outings.
"We had to make a decision with Nate and last night we decided, and the front office decided, that it wouldn't work with what we are trying to do here," Marlins manager Edwin Rodriguez said Wednesday.
The Marlins have not announced who will start in Robertson's place, although Rodriguez said that Chris Volstad could be his choice. Volstad is 1-0 with a 1.38 ERA in two starts with Triple-A New Orleans.
"I like Volstad," Rodriguez said. "I think he should be the first option, but then again we have to wait and see."
Sean West, a 6-foot-8 right-hander who started 20 games for the Marlins last season, is also an option. West is 3-3 with a 3.50 ERA in nine starts with New Orleans.
In other moves Wednesday, the Marlins selected the contracts of right-handed reliever Jorge Sosa and catcher Brad Davis from New Orleans. They also placed catcher Brett Hayes on the 15-day disabled list with a left wrist contusion.