The 22-year-old De La Rosa, who held the champion Giants hitless Friday, is not expected to make the club's Opening Day roster, but Mattingly admits he's coming faster than expected.
"Last spring I never heard Rubby De La Rosa," Mattingly said. "For a guy to hit the radar -- I saw it in L.A. at the development camp and now in live competition -- he makes you open your eyes. All of a sudden he jumps on the radar. It's been quick. You can't really deny what he looks like facing Major League hitters. He could put a smile on your face."
De La Rosa split last year between Class A and Double-A, and was named the organization's Minor League pitcher of the year. The previous year, at rookie ball, he pitched in eight games before being sent home after allegedly having a run-in with a trainer.
2010 Spring Training - null
Sights & Sounds
Spring Training Info
"It's filthy," he said. "He's 97-98 [mph] with a changeup that guys don't see. He has work to do on his breaking ball, but his stuff is legit. You can't teach 98 and the feel he has for the changeup. The breaking ball, at times he doesn't seem to have as good a feel for it."
Mattingly said De La Rosa's progression is now at a critical point.
"He's really still in the spring category where he opens your eyes and you send him down, and he should rip it up and the year could just go bad," he said. "He'll let us know. If he keeps looking like he does now, he'll let us know that he's ready."