Ross, who was feeling "foggy" later that night, has sustained one other concussion in his career, after being knocked over by Mike Cameron in a home-plate collision in 2007.
"Given his position, we can't put him at risk with any additional foul balls or foul tips to the mask," manager John Farrell said.
Lavarnway has been swinging a hot bat with the PawSox, hitting.321 with seven doubles, two home runs, 15 RBIs, 16 runs and 13 walks.
Jason Mastrodonato is a reporter for MLB.com. Follow him on Twitter @jmastrodonato. This story was not subject to the approval of Major League Baseball or its clubs.