DENVER — His velocity is down and his pitches are up.

But Jake Arrieta calmly insists his looming free agency hasn't been a factor in his early struggles, which were magnified Tuesday during a 10-4 loss to the Rockies in the first game of a doubleheader at Coors Field.

“I'm not worried about the contract year,” Arrieta said after lasting only 32/3 innings — his shortest outing in 105 starts as a Cub. “I'm worried about being healthy and taking the ball every fifth day. If I'm able to do that, the results in the long run will play in my favor. I know that for sure.

“Financially, things will work out anyway. I don't put a whole lot of time and effort and energy into my free agency. I'm here now to try to win games for the team. I think guys respect that and teams respect that.”

In the second game, John Lackey provided some stability for the rotation with 10 strikeouts as the Cubs earned the split with an 8-1 victory.

Jeimer Candelario, promoted from Triple-A Iowa for the second game as a 26th player, ripped a single that started a five-run second and capped the rally with a bases-loaded walk.

As for Arrieta, his dominance throughout his 2015 National League Cy Young Award-winning season and his 18-win season in 2016 has vanished. Arrieta has allowed 45 hits in 382/3 innings as his ERA has swelled to 5.35.

In two career starts at Coors Field, Arrieta has allowed 14 earned runs on 22 hits in 82/3 innings.

“I didn't do my job,” Arrieta said after his short outing. “It was a pretty poor performance on my part.”

Arrieta struck out three during his brief stint, and the lack of swinging strikes was apparent to manager Joe Maddon.

“There's something different at the plate,” Maddon said. “They're not missing it as often. That's what I'm seeing. The velocity is not the same, just looking at a (radar) gun.

“I can't tell you specifically (what's wrong). It looked like he threw some good slider/cutters. But typically he gets bad takes, called strikes or swings and misses more often, and that was not there.”

This marked the second time in three starts that Arrieta failed to pitch five innings, but Maddon insisted that neither Arrieta nor the three remaining starters — Jon Lester, Lackey and Kyle Hendricks — has expressed any kind of soreness.

Maddon, however, did say that regaining velocity can be a yearlong struggle, and the returning four starters pitched in all three layers of the 2016 postseason.

“If something was bothering them, I would be more concerned,” Maddon said.

Lackey relied on his breaking pitch to earn his 10 strikeouts through seven innings, becoming the first opposing pitcher to pitch at least seven scoreless innings and strike out at least 10 at Coors Field since Pedro Martinez in 1997, according to ESPN.

The Cubs rotation for this weekend at St. Louis remains uncertain. All signs point to Eddie Butler being promoted from Iowa to start Friday night, with Lester being pushed back to Saturday after a 120-pitch outing Sunday.

But thunderstorms are forecast for Wednesday's series finale at Colorado, which could mean that Kyle Hendricks would start the series opener.

mgonzales@chicagotribune.com

Twitter @MDGonzales