"Gregg came in here and hasn't given up a run all spring," Piniella said Sunday. "I think leaving spring training, we're a better team the way we've put this thing together."
The Cubs obtained the 30-year-old Gregg in an offseason trade with Florida. Last season, he saved 29 games for the Marlins.
The right-hander also had 32 saves in 2007 and has 62 for his career, which began with the Angels.
"That's where I'm comfortable. That's what I've done for the last couple of years," Gregg said. "I'm excited about getting out there in a new year."
Marmol, who also pitched for the Dominican Republic in the World Baseball Classic, entered Sunday with a 1-1 record and 4.50 ERA in spring training. He has hit five batters with pitches.
"We said we'd let both pitchers compete here in the spring, and they both pitched well," Piniella said. "I'm fortunate that I have two pitchers here that I have confidence in that can do that job."
Marmol returns to his role of set-up man to begin the season.
"Disappointed, yeah," Marmol said. "I can't control this."
The Cubs reduced their spring roster to 30 players. They optioned RHPs Kevin Hart and Randy Wells to Triple-A Iowa. C Mark Johnson, INFs Andres Blanco and Esteban German and OFs Brad Snyder and So Taguchi were reassigned to minor league camp.