Beckett, who has been dealing with right elbow inflammation, has been scratched from that start and will visit renowned orthopedist Dr. James Andrews in Birmingham, Ala.
The last time Beckett pitched was on Aug. 17, when the Blue Jays tagged him for eight hits and eight runs over 2 1/3 innings. Two days later, the Red Sox revealed that Beckett had been dealing with some numbness in two of his fingers, and a few days after that, it came to light that elbow inflammation was also plaguing him.
The first target for his return was on Tuesday against the Yankees, then the Red Sox thought he might be able to pitch Friday, and now it's uncertain when he will next pitch.
"It's obviously tough on the team," said lefty Jon Lester, who hurled 6 2/3 innings on Thursday. "He's one of our horses and a guy that we count on for a lot of innings. But, you know what, he has to take care of his body. If his body is telling him that he can't pitch, then he can't pitch. We're going to do our best to pick up his slack. We'll just keep rolling."
Thanks to Monday's off-day, Daisuke Matsuzaka can pitch on regular rest on Friday night. David Pauley, who was recalled from Triple-A Pawtucket on Sunday but hasn't pitched since, is all but certain to fill Beckett's spot in the rotation on Saturday. The only way that won't happen is if Pauley is needed out of the bullpen on Friday.
Beckett was not in the clubhouse after Thursday's 3-2 loss to the Yankees, and therefore, he was unavailable for comment.
"He's just not quite 100 percent," said Red Sox general manager Theo Epstein. "He's getting better. Again, not quite 100 percent. As much for his peace of mind as anything, he's going to see Dr. Andrews on Friday morning."
Epstein said that the numbness in the fingers is no longer occurring, and the problems are strictly with the elbow at this point.
Ever since experiencing arm problems when he was 19 years old, Beckett has had a strong relationship with Andrews, and he has often spoken about the enormous trust he has in him.
"We don't want to put any pitcher in a position where he has to take the mound at anything less than 100 percent," said Epstein. "We think this is the right move. We had him penciled in. I think you could tell from our answers to you guys the last couple of days, this might be a possibility. It's the right thing to do."
In his session with reporters on Wednesday, Beckett admitted how much the elbow problem has been weighing on him.
"It's tough," Beckett said. "I think mentally it's equally as hard as physically. There's some sleepless nights. You're thinking about all kinds of stuff and generally whenever you're dealing with something like this, you end up thinking the worst and not necessarily staying optimistic all the time."
And that, ultimately, is the main reason he's going to see a man he has long confided in.
"We're always concerned for all our pitchers when they're less than 100 percent, but we're pretty optimistic," Epstein said. "This thing is getting better. It just makes sense. Josh has a great relationship with Dr. Andrews; he's an accomplished doctor. We hope he comes back and can take the ball pretty soon."
Beckett is 11-9 with a 4.34 ERA in 23 starts this season.
Ian Browne is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.