In signing Santana, the Braves will lose their first round pick in this year's draft (26th overall). Their compensation pick for McCann is now bumped up to the 31st overall pick. The hope is Santana can pitch well enough to tag him with a qualifying offer and essentially get the pick back next year.
Santana is a bandaid to the current troubles of the Braves rotation. Even as a league average starter, he is an upgrade over what the Braves planned to trot out previously. The biggest risk with Santana is his high volatility in performance over the course of his career. On the plus side, while future durability is never guaranteed, Santana has made 30+ starts in five of his last six seasons, something this team is in current need of.