Lester continued his 30-day rehab assignment Tuesday night by blanking the Columbus Clippers on four hits over five innings. But relievers Craig Hansen, Craig Breslow, Mike Timlin, Manny Delcarmen and Travis Hughes combined to squander a four-run lead as Columbus rallied for a 5-4 victory and deprived Lester of his first win.
The 23-year-old left-hander, who was on an 80-85 pitch limit, threw 50 of 79 pitches for strikes, walked two and matched his season-high for strikeouts with six.
"Early on I struggled with my mechanics and had to battle through a couple of innings, but as the game went on everything got a little tighter and a little better," Lester said. "We want to get better every start and get the pitch count up. This time it just happened to be the curveball that worked well. Next time, it may be the changeup or the fastball."
In his previous start on Thursday against Syracuse, Lester was prohibited from throwing his cut fastball, because Boston's medical staff felt it might have contributed to cramping in his forearm that he suffered on May 2 against Indianapolis.
"I threw two of them tonight and felt fine," Lester said of his cutter. "I'm sure they'll increase the numbers as we go on."
After his latest start, Lester is 0-1 with a 1.24 ERA, seven walks and 19 strikeouts in 21 2/3 innings. He's also allowed only 14 hits.
Lester, whose fastball topped out at 95 mph, said he didn't know if, or when, he would make another rehab start for Pawtucket.
"I have no idea," he said. "I found out about tonight's schedule about two days ago. Hopefully, [the pitch count] keeps going up and I make it through healthy."
If Lester makes another rehab start for Pawtucket, it should come on Sunday at Norfolk. PawSox manager Ron Johnson expressed great satisfaction with Lester's ability to throw his fastball and curveball for strikes.
"I thought his fastball had good finish and was downhill," Johnson said. "He threw some really good breaking balls. His curveball made some really good hitters have some of those front-foot swings.
"Jon's kind of an emotional pitcher. But I think it works to his advantage. He gets into it and he's a competitor. It's an emotional thing but he seems to be composed when it comes to repeating his delivery and his pitches."
Timlin, who's on a 30-day rehab assignment because of right shoulder tendinitis, struggled in the seventh, which began with Pawtucket leading 4-2.
Timlin only threw 13 of 23 pitches for strikes.
Mike Scandura is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.