The Cubs think Jake Arrieta can get even better than he already is.
Yes, seriously - a guy with 18 wins on Sept. 5 and a 2.03 ERA can get even better.
Arrieta is a fitness freak, with an incredible work ethic that has drawn awe from everybody around him (remember Joe Maddon's money quote about Jane Fonda's workout tapes?).
The 29-year-old ace has seen his career take off since being traded to the Cubs in July 2013, but where is the ceiling? How high can he fly?
Even he's not sure.
"I don't know how good I can be," Arrieta said after shutting down the Diamondbacks in Saturday's 2-0 victory. "That's what I'm trying to figure out. That's why I do what I do every day in between starts to prepare myself as best as possible to go out there and see what the results are.
"They've been good. There's some things that I'd like to do better. I like winning for the team. That's the accomplishment, really, for me - getting wins for the team."
Yes, a guy who has given up just four hits over his last 17 innings and has a 0.99 ERA over his last 14 starts has things he would like to do even better.
Arrieta's catcher on Saturday, David Ross, agrees that the sky is the limit for Arrieta having seen the progression the right-hander has made just since spring training.
"He's right there at the top with the best as far as stuff goes," Ross said. "He's only going to get better. He's a No. 1. He's got some of the best stuff in the game. I know that for a fact. I've had to hit off of him before. It wasn't a whole lot of fun.
"He's up there at the top with the group of guys I've been able to catch."
A good example of Arrieta's dominance is his use of a changeup to keep the Arizona hitters off balance Saturday.
Arrieta threw the changeup just 3.4 percent of the time in 2015, but he implemented it in the middle innings against the Diamondbacks just because it was another way to dominate.
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The Cubs saw in the scouting report that Arizona hitters could stay on his breaking ball and cutter more, so Arrieta resorted to the changeup, a pitch he basically only throws during side sessions and bullpens.
But it worked.
"He's got command on both sides [of the plate] with his fastball, he's got command on both sides with his cutter, he's got command on both sides with his changeup and he's got command on both sides with his breaking ball," Ross said.
"It makes it pretty tough [on hitters], but it makes my job really, really easy."