Lester, 35, allowed 10 earned runs -- the second-most in his career -- in an 11-4 loss to the Oakland Athletics at Wrigley Field on Tuesday night. Lester has allowed 22 earned runs in 28 2/3 innings in his last five starts.
"[I] have to figure out a way to right the ship, pick my end up, and do better," Lester said. "Flat-out do better. Our rotation has pitched well, except for me."
Lester is 9-8 with a 4.46 ERA this season. He is on pace to finish with his highest ERA since posting a 4.82 mark with the Boston Red Sox in 2012.
Lester is 186-106 with a 3.55 ERA since debuting with the Red Sox in 2006. He is a five-time All-Star and three-time World Series champion.
Chicago is 61-52 this season. The first-place Cubs have a 2 1/2 game lead over the St. Louis Cardinals in the NL Central Division.