Orioles president of baseball operations Andy MacPhail said Rosario, who turned 21 in September, is a big-bodied, good-armed kid who lacks experience but "captured the imagination of our guys.
"I mean, [he] really has the chance to grow into something. Whether he's ready or not, we'll find out. But [we] couldn't really, didn't feel like we could miss the opportunity to take a chance on [him]."
Rosario spent last season between rookie-level Helena and Class A Wisconsin, and he went 4-0 with two saves and a 4.50 ERA in Class A. He pitched the final inning of a combined no-hitter at Wisconsin on Aug. 24, and totaled 59 strikeouts in 46 1/3 innings.
"I'm very surprised," Brewers special assistant for pro scouting Dick Groch said of the Orioles taking Rosario. "He's a very, very good pitcher, and we obviously hold him in high regard. It's just that that's a quantum leap for anyone."
Interestingly enough, it was the Brewers who selected Orioles reliever Pat Egan in a move that shocked Baltimore.
"That's a dynamic that frankly we didn't anticipate," said MacPhail, who left the door open to possibly including Rosario in future trade discussions. "But [it will be] interesting to see how this spring plays out."
The Orioles also lost right-hander Pedro Beato to the Mets in the second round of the Major League portion of the Rule 5 Draft. In the Double-A phase, the Giants selected catcher Dashenko Ricardo and the Marlins selected right-hander Brent Allar.
Baltimore added three players in the Minor League phase of the Rule 5 Draft. Infielder Dale Mollenhauser (White Sox) and lefty Casey Lambert (Cubs) were added in the Triple-A phase, while right-hander Jacob Rasner (White Sox) was selected in the Double-A phase.
During the Major League phase of the Rule 5 Draft, eligible players left unprotected from their club's 40-man rosters may be selected for $50,000. A player selected must remain on his drafting team's active Major League roster during the following season or be sent back to the original club for $25,000.
"The drafting is the easy part," MacPhail said. "It's the keeping that becomes the big issues."
MacPhail said the Orioles wouldn't have drafted Rosario if they didn't think he had a legitimate shot of sticking.
"He really looks the role," said MacPhail, who was impressed with the video footage he saw of Rosario.
"He's got good stuff. Whether he's ready for this or not, we will find out [in] Spring Training. It's such a cheap gamble, even though the odds are against it. We find ourselves thinking, 'You might as well.'"
Brittany Ghiroli is a reporter for MLB.com. Read her blog, Britt's Bird Watch, and follow her on Twitter @britt_ghiroli. This story was not subject to the approval of Major League Baseball or its clubs.