Even if they don't play another game, Chicago Cubs All-Stars Starlin Castro and Anthony Rizzo have proven they are core players for the organization.

But is that still the case for catcher Welington Castillo?

He was handed the starting job last season and played well on defense while coming on offensively enough in the second half to believe 2014 would be even better. It hasn't been.

Castillo is hitting .241 with 11 home runs and 40 RBIs going into Friday's game against the Pittsburgh Pirates. He's only played in 99 games so far and has just 22 walks which translates to an on-base percentage of .299. It's been a pedestrian year for the 27-year-old who has had some nagging injuries, including a knee issue which shut him down prematurely last sesaon.

But the most telling statistic about Castillo might be what the Cubs pitching staff does when he's behind the plate.

The Cubs' ERA with Castillo catching is 4.27. For comparison, when John Baker is behind the plate, their ERA drops a full point, to 3.26. Castillo has a strong arm, and blame for many of the stolen bases against him can be attributed elsewhere, but that run difference in ERA is more important than anything Castillo can contribute on offense or defense.

In fact, considering the Cubs have pitched a handful of rookies, from starters to relievers this season, a case can be made that Baker, who's hitting just .195, has been more valuable. On almost every occasion the team has trotted out a hurler for his debut it's been Baker who's been behind the plate. He's been the rock for pitching coach Chris Bosio and the staff.

Of course team ERA with one catcher or another doesn't tell the full story. For example, Baker mostly only caught Jason Hammel during the first half of the season, allowing him to develop a rapport with the pitcher. Meanwhile, Castillo had to work with, and figure out, the other four starters. But that's what a good starting catcher has to do on all teams. A contending team undoubtedly wants the complete package from it's starter, or something close at least.

And remember, although Baker has been around longer, Castillo isn't an inexperienced rookie. At some point it has to click and it's unclear if it has or if it will. There is no one who works harder, but calling a game and getting the most out of the pitcher is almost an art not a science.

The Cubs drafted catchers in the first and third rounds in June. With No. 4 overall pick Kyle Schwarber ripping up the minors since joining the organization, there's a chance we're looking at Castillo's replacement someday. It remains to be seen how quickly Schwarber can make it to the big leagues as a catcher -- if he stays at that position -- as his bat is ahead of his glove right now. And there are free agent options as a stop-gap even if Schwarber remains behind the plate and takes some time. ESPN.com's Jerry Crasnick examines one who might be available here.

The Cubs haven't said or intimated they are looking to replace Castillo, but he's no longer a lock -- if he ever was -- to be their long-term starter. Since next season isn't a win-at-all costs year, maybe he's given another chance to grow, especially in terms of calling a game. But with the Cubs' young talent on the mound, they may need the best they can find behind the plate to groom their arms.

It's unfortunate for Castillo his biggest potential weakness might be the most important part of a catcher's game. That's why players such as Baker can survive despite the lack of offense. And with Castillo simply having another average year at the plate, the Cubs could look to replace him behind it.