Actually, they wouldn't have to sign a backup catcher. As of today, they have three catchers on the 40-man roster (Alfaro, Knapp, Rupp), one on the AAA roster (Moore), one on the AA roster (Fisher), three on the Clearwater roster (Bossart, Numata, Rivero), two at Lakewood (Cabral, Grullon), two at Williamsport (Barbier, Lartigue), four on the GCL roster (Duran, Garner, Martinez, Rodriguez), seven between the two DSL rosters, and an additional four Latin American prospects who have yet to be assigned to a roster.
They may want to bring in a non-roster vet or two, just to work in spring training, but they have no shortage of young catchers to fill out the minor league rosters.