After the Marlins beat the Rockies, 10-5, to salvage the fourth game in the series finale at Coors Field, manager Fredi Gonzalez announced some shuffling to the rotation.
Volstad will now join the rotation, and he will get his first big league start on Friday against Los Angeles at Dodger Stadium.
Josh Johnson will be activated off the disabled list this week and start on Thursday in L.A.
Along with adding the two right-handers to the rotation, Ryan Tucker and Mark Hendrickson were switched to the bullpen.
"I think it makes us better as a team, and go forward," Gonzalez said. "[Hendrickson] is fine. He wants to be part of it. He's a pro -- whatever he can do."
The 34-year-old Hendrickson started on Sunday, and he worked four innings before being relieved by Volstad in the fifth. Volstad ended up tossing a scoreless two innings scoreless in his MLB debut, and he was credited with the win.
Volstad, the 16th overall pick in the 2005 First-Year Player Draft, had his contract selected from Double-A Carolina on Sunday afternoon. A 6-foot-7, 21-year-old, Volstad was 4-4 with a 3.36 ERA in 15 starts for Carolina. The right-hander, who is from Palm Beach Gardens, Fla., has already logged 91 innings this season. He's struck out 56 while walking 30.
The Marlins promoted Volstad the day after they optioned lefty reliever Taylor Tankersley to Triple-A Albuquerque.
Hendrickson has a 7-7 record with a 6.24 ERA in 19 starts. The lefty, who has been a reliever in the past, has logged 101 innings this season.
He is accepting his new role in the bullpen, but he pointed out that he's gone through a tough few days, perhaps anticipating the switch.
"It's been a tough probably three days here. Mentally, I'm pretty happy with how I handled it," Hendrickson said of being able to focus on his Sunday start. "In the game, I felt like I was out there competing, when [Gonzalez] told me I wasn't going out there in the fifth, I was upset."
Hendrickson, the Marlins Opening Day starter, felt he was competing and he wanted the chance to continue.
"For me, the urgency is to go out there and win now. Sometimes, certain things happen in this game that you don't know," he said. "Obviously, they've got some young talent here. Maybe they want to take a look, and that's out of my control.
"I'm motivated on winning now. We're hanging around and putting ourselves in position to do that."
The first of five first-round picks for the Marlins in '05, Volstad is a sinkerball pitcher who has excellent control. He did have a slight right arm ailment that caused him to miss one start for the Mudcats.
The Marlins will shake up their rotation for the rest of the road trip. Johnson, who pitched what will be his final rehab assignment game for Double-A Carolina two days ago, will see his first big league action on Thursday since July 4, 2007.
Johnson is returning from Tommy John elbow ligament replacement surgery to his right elbow last Aug. 3. Scott Olsen will start on Wednesday against the Padres at PETCO Park.
Ricky Nolasco is pitching on Monday in San Diego, and Andrew Miller will go on Tuesday.
Joe Frisaro is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.