After catching the foul ball on the carom, Swihart went to the ground in pain.
"Yeah, it wasn't pretty," Red Sox manager John Farrell said. "You get in up against that concrete base there, the base of the wall, and we don't know exactly what has taken place yet. He is going to go for an MRI here tonight. Until we get the results, it's hard to really comment any further on that."
Hanigan left the game at the start of the seventh with a neck strain, an injury he had also dealt with a couple of weeks ago. Chasing Steven Wright's knuckleball to the backstop on several occasions couldn't have helped.
The Red Sox are confident he doesn't have a concussion.
"It's more tightness, some pain, but it's not any dizzying type," Farrell said. "He had it that one evening. But every field test, every training-room test to determine that has been negative."
Hanigan also underwent tests after Saturday's game. The problem for the Red Sox is that if Hanigan misses any significant time, Swihart would be the obvious candidate to replace him as the backup catcher.
"Once we get information on them, we'll have a better read on the severity of both," Farrell said.
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.