Slugger David Ortiz was discharged from Massachusetts General Hospital in Boston on Thursday after he underwent tests on his heart.
Ortiz said he has been cleared of any heart abnormalities.
"It's not my heart," Ortiz said. "The doctors ran all sorts of tests on me and my heart was not the problem at all. It was the lower part of my right pectoral muscle that, for some reason, was shaking. It scared me because I had never felt anything like that in my chest. But after all the tests, they said my heart was fine."
Ortiz, who has been prescribed medication to calm the non-life-threatening activity, was told to stay at home Thursday night, when the Red Sox open a four-game series against the Toronto Blue Jays.
He expected to be at Fenway Park on Friday to work out with his teammates and said he could play as soon as Saturday night.
Red Sox spokesman John Blake said later Thursday that it's "very premature" to think Ortiz would play Saturday. Manager Terry Francona was also cautious about putting Ortiz back in the lineup.
General manager Theo Epstein echoed those sentiments after after Boston's 6-4
win over Toronto, saying that "the report's not necessarily accurate and
there's no update tonight on David Ortiz."
"I talked to David today," Francona said before Thursday night's homestand opener against Toronto. "I think he was very upbeat. I think the medical people are also, but, at the same time, we sent him home to be careful. So to deviate from that plan now would be, in my opinion, stupid, and we're not going to be stupid."
Ortiz was not in the clubhouse before the game.
He returned to Boston from the team's West Coast trip for evaluation after he felt heart palpitations before Monday night's game in Oakland. It was the second time in August that Ortiz was treated for the problem. He said he went through medical tests, and doctors determined his heart palpitations were related to stress and fatigue.
"I sat in that MRI tube for almost three hours and they showed me all the test results. Thank God it's nothing serious," Ortiz said. "Now that I know I'm OK, I'm ready to get back out there and help the club."
The Red Sox have been reeling in recent weeks. Boston has lost 21 of 29 games in August, a stretch that includes 12 losses in its last 14 games, to slide eight games behind the first-place New York Yankees in the AL East entering Thursday's games.
With 29 games remaining on its schedule, Boston also trails the Chicago White Sox by 7½ games for the wild-card lead. Ortiz leads the American League in home runs (47) and RBI (121).
The Red Sox returned to Boston on Thursday after losing six straight.
Left fielder Manny Ramirez has missed several games on the trip with a knee injury, and Francona did not put a timetable on when he will return. Center fielder Coco Crisp injured his shoulder making a diving catch Tuesday but batted leadoff Thursday.
Catcher Jason Varitek, shortstop Alex Gonzalez and outfielder Trot Nixon are on the disabled list. All were expected to make rehabilitation assignments with Triple-A Pawtucket: Nixon on Thursday night, Varitek starting Friday and Gonzalez on Saturday. Francona said Varitek could start Monday against the White Sox.
Nixon was 0-for-4 with two putouts in seven innings Thursday night.
"I felt fine in the field but I was not comfortable at the plate. My timing is off. My timing at the plate will determine when I can join the big club," he said in a statement released by the team. Nixon declined to speak with reporters after Pawtucket's 9-1 loss to Ottawa.
Nixon was scheduled to continue his rehab assignment Friday, joined by Varitek; Gonzalez was expected on Saturday. Francona said Varitek could start Monday against the White Sox.
Outfielder Wily Mo Pena received a cortisone shot on the injured wrist that has sidelined him and could return in "a couple of days," Francona said.
Information from The Associated Press and ESPN's Pedro Gomez was used in this report.