After starting the year 6-0 with a 2.72 ERA, Lester has just one quality start in his last seven outings and has recorded a 7.30 ERA.
"We've got to get Jon Lester back on track," Farrell said. "There's no denying that. He'd be the first to admit to you as well."
Perhaps just as concerning, Lester, who prides himself on pitching deep into games, hasn't made it through six innings in his last three starts. That would spark health concerns for some, but Farrell said Lester's issues are more mental than physical.
"He's sound healthy," Farrell said. "I think he's doing a typical job of repeating his delivery. I think it's the consistency of execution, particularly with two outs when you get in a situation with men on base. There's been, I think, at times he'll rush a little bit to try and get through a given inning, and that might cause that inning to be prolonged."
Lester is scheduled to start against the Blue Jays on Thursday night, and Farrell is confident he'll get back on track.
"I don't think Jon Lester's stretch he's on will continue," Farrell said. "He's too talented of a pitcher."
Michael Periatt is an associate reporter for MLB.com.