The 40-year-old right-hander was sent back to Boston from Atlanta on Tuesday for an exam after he got roughed up in his last two starts. He was then examined by Red Sox medical director Thomas Gill.
Schilling nearly pitched a no-hitter at Oakland on June 7, giving up a two-out single to Shannon Stewart in the ninth inning. But he allowed 11 earned runs and 19 hits in 9 1/3 innings in his next two starts.
On Monday, Schilling, 14th on the career strikeout list with 3,086, failed to fan a batter -- the first time that's happened in one of his starts since July 1, 1993.
"I have not felt right this year," Schilling told WEEI-AM earlier Wednesday.
"Pain is a relative term. There's been a lot of this year trying to discern what's because I'm 40 or what's because I don't feel good," he said.
"He's fought some tendinitis," said Red Sox manager Terry Francona. "As far as labeling [the injury], I don't know that I can."
Schilling had a conference call on Wednesday afternoon with Francona, general manager Theo Epstein and Dr. Gill.
With Schilling out of the mix for the next couple of weeks, the decision then was made that Josh Beckett will pitch Sunday, when the Red Sox play at San Diego. He will be on regular four days' rest because of an off-day on Thursday. Julian Tavarez is scheduled to start Monday at Seattle. The Red Sox have listed Tuesday's starter as TBA.
Francona refused to speculate on whether Tuesday could mark the return of left-hander Jon Lester, who was recently optioned to Triple-A Pawtucket and hasn't pitched in the Major Leagues since being diagnosed with anaplastic large cell lymphoma last August.
Lester pitched for Pawtucket on Wednesday in a game at Indianapolis, giving up five hits and three earned runs over five innings. He walked four and struck out three. Lester, who threw 92 pitches, is 1-3 with a 2.49 ERA in nine starts for Pawtucket.
But the focus on Wednesday was Schilling.
"The good news is, structurally nothing has changed on [Schilling's] MRI, which is really good because he's thrown a lot of innings," Francona said. "And when you see the dropoff the other night, it certainly sends up a red flag."
Schilling tore his labrum while playing for the Philadelphia Phillies in 1995. He said when that happened, he went from throwing 95 mph in one inning to 80 mph in the next. He felt fine during that game but woke up in pain.
He said Monday night's outing "eerily similar" in that his velocity took such a big dip, but the difference this time was he did not wake up hurting.
Schilling will be examined by Dr. Gill again on Friday and will then fly out to San Diego and do his rehab under the watch of the team's training staff.
"The ball wasn't coming out real well," Francona said. "And when he was reaching back, I think what was happening is he was losing command and not really getting the velocity. I don't think that's a good combination for anybody."
In fact, Schilling topped out at 90 mph in Monday's start and most of his fastballs were in the mid 80s. Before the velocity became such a glaring issue, Schilling's big problem this season was consistency.
"What seems to be happening at times is in following up one of his good starts, the consistency is not quite there," Francona said. "We want to get him back and have him get a chance to be Schilling and be consistent and not have a bad one and a good one."