Wilson came into this season as the eighth-inning setup man. League, who came into last season as the closer, came into this season without a defined role.
Mattingly concedes he's "concerned with Wilson's success," convinced, he said, that the right-hander's twice-repaired elbow is sound.
"But compared to last year, he's been a lot more stressful," Mattingly said of Wilson's outings. "Last year, it was clean, clean, clean. This year, it hasn't been the same. Yet. I really believe he'll get there. At this point, he's not."
So, does that mean League has earned a promotion to the back end of the game?
"Slowly, but surely, it's changing," Mattingly said of League's role. "The more you send a guy out there and the more outs he gets, you gain confidence in him."
Chris Withrow, Chris Perez and J.P. Howell also have been getting some action setting up closer Kenley Jansen.
Ken Gurnick is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.