MESA, Ariz. -- Chicago Cubs righty Jameson Taillon may not be ready for Opening Day as he'll undergo tests on his back after leaving Saturday's spring training start during pregame warmups, manager Craig Counsell announced Sunday morning.

"Came in today pretty much the same as he left yesterday," Counsell said Sunday. "Probably not the improvement we were hoping for.

"There's a level of concern for Opening Day."

Taillon, 32, has not appeared in a Cactus League game -- Saturday was to be his first -- but he has been throwing without issue in bullpen sessions and on the back fields at spring training. He's coming off a down year in 2023 (in which he went 8-10 with a 4.84 ERA) after signing a four-year, $68 million deal with Chicago.

Counsell was asked about the severity of the ailment.

"I don't think it's a long-term absence for Jameson, so don't think it's one of those concerns," he said. "We're in good shape. We have a bunch of guys stretched out and see how that plays out."

The Cubs have internal options to replace Taillon, including veteran Drew Smyly and youngsters Hayden Wesneski and Javier Assad.

Taillon fit into the middle of the team's rotation with ace Justin Steele set to start on Opening Day, while newcomer Shota Imanaga sits behind him along with righty Kyle Hendricks. Left-hander Jordan Wicks rounds out the rotation, which is down a man for the time being.

"We're OK for now," Counsell said.

Taillon has a career 4.00 ERA over seven major league seasons, including time with the New York Yankees and Pittsburgh Pirates before spending last year in Chicago.