In addition, infielder/outfielder Brock Holt was sent home from the ballpark before the start of the game. Lefty reliever Robbie Ross Jr. is also ill.
Last week, first baseman Mitch Moreland was quarantined for three days and couldn't join the Red Sox for exhibition games in Washington, D.C., and Annapolis, Md., against the Nationals. Hitting coach Chili Davis, assistant hitting coach Victor Rodriguez and trainer Brad Pearson are others who have been impacted over the last several days.
Moreland tested positive for the flu before he was quarantined. Betts, Holt and Ross tested negative on Wednesday and are just dealing with flu-like symptoms at this point. But the club will continue to monitor them.
"This has hit our clubhouse fairly hard," said Red Sox manager John Farrell. "It's been popping up all Spring Training. We've had coaches leave Annapolis and come back [to Boston] early -- we've quarantined a number of different players but it's hanging around. We still have to let them get through their symptoms before they get back to active play."
Chris Young got the start in right field in place of Betts on Wednesday.
Ian Browne has covered the Red Sox for MLB.com since 2002. Follow him on Twitter @IanMBrowne and Facebook. This story was not subject to the approval of Major League Baseball or its clubs.