Rosario did not make anyone forget Helton in his seven innings, but there were no mishaps.
"I was a natural," Rosario said with a wide grin.
The first grounder of the game, by Jose Tabata to shortstop Troy Tulowitzki, was a bit of an adventure. He seemed to just be watching the two-time Gold Glove shortstop do his thing before it dawned on him that he needed to cover first for the throw.
Rosario also made an awkward flip to first after fielding Jeff Locke's groundball.
Offensively, Rosario scored the winning run on third baseman Nolan Arenado's double in the seventh, then was replaced by Helton.
"It was funny watching him play over there," Arenado said. "He made a play in the hole and flipped it instead of throwing it. But, hey, he made his plays; he did what we wanted him to, so it was all good."
Manager Walt Weiss was happy Rosario made it without errors or injuries.
"He's got good feet and is a good athlete," Weiss said. "He did a nice job."
What he lacks in experience at first base, Rosario makes up for in enthusiasm. He was beaming while recalling that Helton told him he did well.
But asked if it was better to have an enthusiastic novice than an apprehensive one, Arenado thought for a second then picked an alternative answer.
"I'd rather have Todd," he said.
Thomas Harding is a reporter for MLB.com. Read his blog, Hardball in the Rockies, and follow him on Twitter @harding_at_mlb. This story was not subject to the approval of Major League Baseball or its clubs.