Boston's ace had difficulty sleeping Sunday night because of his back.
"I was a lot more optimistic yesterday than I am today," he said.
Red Sox manager Terry Francona, who was in Port St. Lucie, Fla., with the team for a contest with the Mets, also reported Beckett's aches and pains.
"We just have to get the spasm out of there and let it calm down," said Francona. "When we do that, we'll allow him back to doing some activities. But, thankfully, I think he's resigned himself to do the right thing, not to do something silly that maybe four years ago he would have."
Beckett was 20-7 -- the Majors' only 20-game winner -- with a 3.27 ERA last season. This spring, he has faced Northeastern University in an exhibition contest and the Twins in a "B" game. He has yet to face a big league lineup, leaving the mound Saturday after making several warmup pitches before the Sox faced the Marlins.
Beckett is unsure if he will be ready for Opening Day on March 25 against the A's in Tokyo.
"If I would have had as much progress from yesterday to today as I had from the first day to the second day, I wouldn't have ruled that out," said Beckett, who admitted not to be in the best of moods. "Today, I'm just holding up my obligation [in speaking to the media]. It probably would have been better for me to talk to [the media] yesterday, because I was a little more optimistic."
Beckett said a decision on making the trip to Japan -- including a plane fight of approximately of 17 hours -- has not been made yet.
"We haven't discussed any of that," he said "We're just doing what we're saying, going day to day on how I feel. I pretty much have an interview with two or three doctors every day and I report to the manager or the pitching coach and my trainer."
Francona said there's no reason to rush a decision on whether Beckett will pitch Opening Day.
"It just doesn't make sense," he said. "I don't have the answer.
"We usually err on the side of caution. We usually try to use good judgment. We're going to do what's in the best interest of our ballclub, short-term and long-term. We always do. I can't imagine a 17-hour flight would be good for anybody. But that's just the way it is. So you deal with it, and we'll be all right."
Beckett, who has had no history of back problems, said he has undergone tests, including an MRI, which point to a muscle, rather than skeletal, issue.
"We've done all the tests and everything like that," he said. "I think there's nothing wrong with my discs and I think that's just what they were making sure of. There's nothing wrong with the discs. So it's definitely just a strained muscle or a pulled muscle or whatever you want to call it. We just get it to heal when it heals."
Beckett said he was unsure when he would be able to throw again.
"We're going to make sure it's completely pain free," he said. "If I lay in one position too long, or sit in one position too long, or stand in one position too long, it just kind of goes backwards. So I keep moving and stuff like that.
"It's kind of numb right now. I've had like six or seven ice packs on it all day."
He has not received any injections to reduce inflammation.
"I kicked some of those ideas around because it's a tough time during Spring Training, trying to get everything going," he said. "You don't really want setbacks this time of year. They basically told me it would be iffy if they could find a spot to put the shot in there."
While it was believed Beckett may have lost his footing on loose dirt on the City of Palms Park mound, the pitcher refused to speculate -- or blame.
"I'm not going to get into the blame thing," he said. "Everybody's doing a job here. Nobody's job's easier than anybody else's. I'm not getting into that."