CHICAGO -- Cubs starter John Lackey left Sunday's game against the St. Louis Cardinals in the seventh inning with shoulder stiffness, but Lackey says he'll be just fine.

"I'm OK," Lackey said after the 6-4 loss. "The extra bullpen session was probably not the best for me. I've been feeling a little tight since then. Trust me, I've been out there feeling worse. I'll be alright."

Twice in the past two weeks -- including Thursday -- Lackey has warmed up in the bullpen late in games as the Cubs played in extra innings, but neither time did he enter the game.

On both occasions, Lackey had already thrown his side session between starts, adding more pitches to his day, which led to his early exit Sunday. The 37-year-old righty said he'd be fine.

"An old thing," he joked. "It was pretty tight. I feel like I could have gotten through that last hitter. Joe [Maddon] wanted to err on the cautious side. We'll be alright."

Lackey knelt down while grimacing after throwing a pitch to Randal Grichuk in the seventh inning Sunday. The pitcher threw one more before Maddon pulled him.

Lackey made 33 starts last season, tying for the league lead, and led the Cubs with 24 this year, including Sunday's game, in which he pitched 6.2 innings. He gave up four hits and one unearned one while striking out five and walking one.