With Mike Napoli and Shane Victorino both going on the disabled list in recent days, Boston was a little thin from the right side of the plate.
Lavarnway has split his playing time between first base and catcher this season.
If he starts any games during his stint with the Red Sox, it will likely be at first base.
"We felt that we needed another right-handed bat, particularly in these two games [without the designated hitter], where there's going to be some pinch-hit opportunities when you get down to that pitcher's spot," said manager John Farrell. "But I think more than anything, just to balance out the left-handers that we have on the roster currently and to get some balance to it."
This is the fourth straight season Lavarnway has spent time with the Red Sox, and he had a handlebar moustache upon his arrival this time.
For Pawtucket, he is hitting .265 (43-for-162) with seven doubles, two home runs, and 11 RBIs in 44 games.
Lavarnway found out about the move before Sunday's game in Syracuse.
"They called me off the field during BP [Sunday] said your cab is in five minutes," said Lavarnway. "I'm excited about the direction I'm headed. First base is going along nicely, first base is coming along. I'm very excited."