Castonzo injured his medial cruciate ligament in the Colts' pre-season clash with the Cleveland Browns and could miss their season opener against the Oakland Raiders on September 8.
Pagano told reporters: "The MRI was pretty much negative. It wasn't as bad as anybody anticipated.
"You never want to lose anybody, and particularly the guy who's protecting the blind side of your quarterback."
Castonzo was drafted 22nd overall in the 2011 draft.