Only it's not that simple, in the case of Mathis and Napoli.
Scioscia goes with his instincts in making that call on any given night. He is emphatic about putting defense first behind the plate, which plays to Mathis' strength. Napoli, a born hitter who catches, has struggled at times defensively this season.
"First and foremost, he's starting to work some things out on the defensive side," Scioscia said before Napoli went behind the plate on Friday night to catch Joe Saunders against the White Sox. "He's getting better. On the offensive side, he's got a little longer swing, and he's going to tend to be more hot and cold than some guys."
Napoli is in a 2-for-30 spin, his average slipping to .275. Mathis is batting .213, but his raw defensive numbers are superior to Napoli's, especially in the category of pitchers' ERA.
When Mathis is behind the plate, Angels pitchers own a 3.98 ERA. Their collective ERA is more than a run higher -- 5.20 -- with Napoli catching. For their careers, the gap narrows. Mathis is at 3.94 in 222 starts, while Napoli comes in at 4.41 in 305 starts.
In terms of throwing out basestealers, it's a wash. Both catchers have a 23 percent success rate this season, with Napoli slightly better lifetime at 23 to 22 percent.
"It's not an end-all," Scioscia said of catchers' ERA, "but it is an important stat pitcher to pitcher. You can evaluate how guys are working with different catchers.
"Jeff Mathis is having a terrific season. You can't look at offensive numbers and say this guy's having or not having a good year if he's a catcher. His most important role is always going to be his defense. That pitcher-catcher relationship is first and foremost."
John Lackey's recent run of excellence -- one earned run in his past 26 innings -- has come with Mathis catching. For the season, the ace has a 2.45 ERA in 95 2/3 innings with Mathis compared to 5.34 in 59 innings with Napoli.
Mathis has handled 129 1/3 of Jered Weaver's 186 2/3 innings. Weaver has a 3.34 ERA with Mathis, 4.71 with Napoli.
Saunders, with Napoli handling about two-thirds of his innings, has a 5.28 ERA with Napoli, 4.32 with Mathis.
Scott Kazmir, the left-handed hammer added to the rotation courtesy of the Rays, has a 1.35 ERA in his two starts -- both with Napoli.
The Angels are 43-30 when Napoli starts behind the plate and 41-25 when Mathis gets the call.
The American League West leaders have flourished (11-5) with Napoli in the designated-hitter role. He's batting .367 as a DH with a .617 slugging percentage, compared to .260 and .469, respectively, when he catches.
Lyle Spencer is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.