CHICAGO -- Is Jake Arrieta all right?
Simply put, that's the biggest question for the Cubs as they head into the second half of the season, well positioned to justify their status as World Series favorites.
It's easy to imagine them competing favorably against the best teams in the Major Leagues if Arrieta can approach the epic level of performance when he was at his best. For almost a full year -- from June 21, 2015, through May 31 -- he was 25-1 with a 1.09 ERA.
Arrieta no-hit the Dodgers in Los Angeles and the Reds in Cincinnati, and for good measure, he annihilated the Pirates in the National League Wild Card Game last October. You don't worry that much about Clayton Kershaw, Madison Bumgarner, Max Scherzer or Noah Syndergaard when Arrieta is strong and full of self belief, going at hitters with his death stare and electrifying slider.
But Arrieta has been mortal since about the time the ivy started flourishing on the Wrigley Field walls, going 3-4 with a 4.81 ERA in his past seven starts.
"It's a confidence issue for me right now,'' Joe Maddon said on Monday night, before Jon Lester and Anthony Rizzo carried the Cubs to a 5-1 victory over the Mets.
Arrieta lost to the Mets' Syndergaard in Game 2 of the NL Championship Series last season. He faces him on Tuesday night at Wrigley.
This national showcase is a golden opportunity for the 30-year-old to reassert his dominance over hitters and turn the conversation back to the Cubs' need to extend his contract before he reaches free agency at the end of 2017.

So we're back to that one question: Is Arrieta all right?
Arrieta admitted he was feeling tired during his trip to the All-Star Game, where he didn't pitch. The Cubs slotted him into the back of their rotation, not the front, so he'll be working on 10 days' rest Tuesday. That's a little more time off than he'd have had skipping one start.
There have been no signs that Arrieta is injured -- no drop in velocity (his sinker has averaged 95 mph in the past seven starts, almost exactly the same as at this time in 2015), no mound visits with a trainer in hand or signs Maddon has babied him. But here's something for you conspiracy theorists.
Arrieta is throwing his slider a lot less than normal. At the least, that seems worrisome.
Arrieta told MLB.com's Carrie Muskat he didn't pick up a baseball for four days during the All-Star break. He cited a need for "mental down time'' and said that his most recent bullpen workout was the best he's had all season.
Keep an eye out for that slider.
According to the Brooks Baseball calculations, Arrieta has thrown 14 or fewer sliders in four of his past nine starts. He hadn't thrown fewer than 17 sliders in a game since May 27, 2014. He had averaged 30.6 in a stretch of 63 starts before that average dropped to 17.7 in his past nine starts.
Arrieta's slider was almost unhittable early this season, but it has been hit unusually hard at times since then, which might explain why he isn't throwing it as often. But without that slider, he's not the guy who compiled the historic 0.75 ERA in the second half of last season.

Maddon did not get into specifics about Arrieta's downturn, but he conceded Monday that he's been concerned enough to go to the Cubs' stable of research and development analysts for help in spotting negative trends.
"I looked into some stuff,'' Maddon said. "I had some questions for our geeks, and they gave me some stuff back. It's really funny. You think observationally you know what's going on, and I presented my observations and I was absolutely wrong. I always find that interesting.''
Without his good slider, Arrieta has been allowing more fly balls. His ground ball-to-fly ball rate was 1.42 in his first 11 starts and has been 0.98 in his past seven.
Arrieta has had more balls hit hard -- his line-drive percentage has increased from 23 to 32, per Brooks Baseball -- and may also have been a little unlucky. His BABIP was .213 in his first 11 starts and is .340 in his past seven.
Arrieta has been throwing marginally fewer strikes during his downturn -- 62 percent, down from 65 percent -- which has meant an extra walk every nine innings. Maddon believes he'll get himself back on track when he locates his fastball more consistently.
"It really comes down to the fastball, command of that fastball,'' Maddon said. "When he gets that rocking, everything else plays off that. If you're not in good counts, if you're not 0-1, they don't have to honor the other stuff -- the slider and the curveball, whatever.''
Lester's outing against the Mets was the third quality start for the Cubs' rotation in four games since the All-Star break, and John Lackey was positioned for one until an eighth-inning homer by Ian Desmond. This is a very encouraging trend for a first-place team that limped into the All-Star break.
But the start that counts the most is the one that only Arrieta can deliver. The Cubs have a lot riding on him getting back on track.