NEW YORK -- It took Cubs closer Craig Kimbrel a long time to learn, develop and hone the knuckle-curve that he uses as a swing-and-miss weapon. Rookie reliever Rowan Wick learned it during Spring Training and is still gaining trust, feel and consistency with the pitch.

Yu Darvish asked Kimbrel about his curveball grip and approach about a week ago, began toying around with the pitch and is already using it in games to collect strikeouts.

"He walked up to me the other day and he was like, 'Hey, I've been working on that,'" Kimbrel said on Wednesday at Citi Field. "I was like, 'Cool.' I didn't know he was going to go out and throw it. I thought that was pretty cool."

Darvish now has three curveball variations. The right-hander has a slow curve that drops under 70 mph and a regular one that sits around 75-76 mph. In his past two starts, Darvish started mixing in the harder knuckle-curve, which comes in around 80-82 mph and has a sharper downward break.

The best example came in the eighth inning on Tuesday night, when Darvish generated a swinging strikeout of New York's Jeff McNeil with the new offering.

"He's just a talented human being," Wick said with a laugh. "If you throw a curveball already, you have a feel for how the ball spins. I feel like once you take your index finger off the baseball, it frees up the way the ball can spin off your hand.

"You're releasing it above your head almost, so if that finger's on there, there's more area for the ball to spin sideways. When it comes off your middle finger, then it's just true, down. Obviously, Darvish played around with it a little bit."

For those counting at home, the new curveball is the 10th pitch that Darvish has featured this season. The starter throws a four-seam and two-seam fastball, plus a slider, changeup, splitter and three types of curves. Darvish also throws a cutter and what the Cubs call a hard cutter.

Kimbrel was amazed that Darvish -- after only minimal practice -- was confident enough in the knuckle-curve to use it in a game so soon.

"Most guys don't trust it that much to be able to throw it and then bring it in a game like that," Kimbrel said. "That just says more about him than anything."

Cubs manager Joe Maddon agreed.

"It doesn't happen," Maddon said. "As a Major League pitcher, I know they experiment with stuff all the time, but to take a pitch like that that requires a lot of feel, from a teammate, and just bring it into a game that quickly and effectively, that doesn't happen."

Rizzo still sidelined

Before the series against the Mets began this week, Maddon knew there was a chance that first baseman Anthony Rizzo might miss all three games due to his ongoing back tightness. It now looks like that will be the case.

On Wednesday, Rizzo was not optimistic that he would be able to return to Chicago's lineup before Friday at the earliest. The first baseman, who said the pain "grabbed out of nowhere" on Saturday, planned on testing his back with some swings in the batting cage.

"It's getting there. Hopefully a couple days, I'll get it to calm down and just manage it," Rizzo said. "It's always frustrating -- it doesn't matter when it is. It's not fun watching from the bench when you can't contribute. But it's just part of it. You just get through it."

Maddon did not entirely rule out having Rizzo available off the bench on Thursday in New York.

"You never know," said the manager. "Even if he's available to pinch-hit tomorrow, that'd be a win somehow. We'll see. I'm not counting him out of it yet."

Worth noting

• The Cubs will send their top two prospects -- shortstop Nico Hoerner (Per MLB Pipeline, No. 1 on the Cubs' Top 30 list and No. 48 on MLB’s Top 100 list) and catcher Miguel Amaya (No. 2 and No. 88) -- as part of the group heading to the Arizona Fall League to play for the Mesa Solar Sox.

Also heading to the AFL for the Cubs will be righty Erich Uelmen (Cubs’ No. 24 prospect), lefty Jordan Minch, righty Keegan Thompson and first baseman Jared Young.

• All-Star catcher Willson Contreras (10-day injured list, right hamstring) went through more running and agility drills on Wednesday -- one day after catching Jon Lester's bullpen session. Maddon said there is still no firm date for a Minor League rehab assignment, but noted that this weekend or early next week was possible.