"He felt it on one of the swings in that first at-bat. So we'll have more tomorrow," manager John Farrell said.
Lavarnway had just the one at-bat in Boston's 4-3 win and made four putouts in his debut at first.
"It was on one of the balls I fouled straight back in my first at-bat," said Lavarnway. "I felt a pain in my hand/wrist area. I thought I would be able to play through it. Tried to stay in the game, obviously, tried to play through it. When I got in the on-deck circle for my next at-bat, when Grady [Sizemore] ended up making the last out, I was having a real difficult time swinging. So I wanted to let a healthy guy have that at-bat instead of trying to force my way in and giving them an easy out."
While his natural position is behind the plate, Lavarnway gained comfort at first base in Spring Training and played 20 games there for Pawtucket this season.
With everyday first baseman Mike Napoli on the disabled list, Mike Carp and Daniel Nava have also seen time at the position.
Ian Browne is a reporter for MLB.com. Read his blog, Brownie Points, and follow him on Twitter @IanMBrowne. Quinn Roberts is a reporter for MLB.com This story was not subject to the approval of Major League Baseball or its clubs.