The youngster from the Dominican has thrown only 183 career innings, most in rookie ball. When he pitched 32 innings for Wisconsin of the Single-A Midwest League last summer, those were his first career innings above rookie-ball level.
The odds are against any Rule 5 player sticking with his new big league club all season. Rosario, with his lack of experience, is even a longer shot than most. Playing at Wisconsin is the equivalent of an O's farmhand being with Delmarva.
Still, the O's felt the $50,000 price was a risk worth taking. If Rosario doesn't stay on the Orioles' 25-man roster for the entire season, they have to offer him back to Milwaukee for $25,000, half the price it cost to draft him.
Lee MacPhail, the O's director of pro scouting, who was a key person in the club's Rule 5 draft prep, said manager Buck Showalter was fully in support of this move.
"You need your manager to be on board with a move like this. If he is not, it's too great of a risk and could limit his ability to win daily, which is the objective," MacPhail said. "(Showalter) understands the risks associated. One of the things that maybe helped influence him was we had some video for him to see. He knows it will be hard for (Rosario) to make the club out of spring and then survive the whole season. But Buck said he would do his best to use him in long man situations and try to keep him from sitting and not being used for two weeks at a time."
Last season, Rosario went 1-0, 1.26 in five games in rookie ball and then was 4-0, 4.50 in 32 innings for Wisconsin. In 56 1/3 innings between the teams, he walked 18 and fanned 59.
He has played pro ball since 2007 and in four seasons, the 6-foot-4, 180-lb.hurler is 12-13, 4.82 with 214 hits allowed, 77 walks and 165 strikeouts in those 183 innings. He has pitched in 59 games, making 15 starts.
"He is a young, projectable, right-handed pitcher who is inexperienced. But he has a good body and a good, live arm. He has an assortment of potentially above average Major League pitches. He is kind of what you look for in terms of a classic pitcher body, arm action and delivery. He has the potential for a very high ceiling," MacPhail said.
"We've seen him 91 to 94 (mph). During the summer we saw him also throw a curve and change-up. This fall, we saw him with a slider. Our people feel the slider has a better chance right now than the curve and his change needs some work. We see him in relief now, but down the road he could be a starter.
"We had full blessings from Buck to roll the dice on this one. You need to put him in spots to keep him viable and developing, but also not totally hamstring our major league effort where it is like you are only carrying 24 players. He had to be on board and he was."
While some clubs drafted players in the Rule 5 that are clearly closer to the majors than Rosario, the O's felt, for what is basically a gamble of $25,000, that they would take the chance on a pitcher they liked.
Will he be ready for the majors next year? Most likely, no. But maybe he will be that rare player that sticks around all year.
"We all kind of agreed this might be an exception. The odds are against him. It's a risk-reward thing," MacPhail. "Having seen some of the younger, deep projection arms that have been in spring camps (all over baseball), I've seen far cruder, far riskier, and far less ceiling guys. From a scouting standpoint, we went in fully armed with information on this guy, even though he has pitched so few innings."