The Colts' defense is just kind of average, as our friend Josh Wilson at Stampede Blue noted. That means the loss of one of their very best players against the Falcons is bad news for them, and good news for Atlanta.
Mike Adams is a 12 year veteran who apparently was saving some of his best football for the end of his career, and he's been an absolute terror in 2015, flying all over the field and picking five passes thus far. I don't have to tell you that taking a player of that caliber off the field can only help a Falcons passing attack that has had nearly two weeks to rest and figure out what the hell is wrong with itself. In his absence, the Colts will trot out either Colt Anderson (seriously, that's his name) or Winston Guy, both of whom can safely be considered major downgrades.
Your obvious counterpoint here is that injuries have not helped the Falcons out significantly over the past several weeks, and that's a fair point to make. My counter-counterpoint would be that every bit helps, and without Andrew Luck and now Adams, the Colts are a weaker football team that has to win on the road. You shouldn't hand Atlanta anything at this point, but hopefully they can take advantage of the matchup they're being presented with.