Colorado selected Alfaro's contract from Triple-A Albuquerque on Thursday.
Brian Serven has been optioned out and Alfaro will take over a share of the backup role behind primary catcher Elias Diaz. Alfaro, 30, has slashed .323/.366/.524 with seven home runs and five stolen bases in 46 games this year between the Triple-A affiliates of the Red Sox and Rockies. Beyond backup catching duties, he can also play some corner outfield and cycle in at designated hitter.