Last season the Colts lost several key skill position players, and unsurprisingly the passing offense struggled. With Addai in contractual limbo until a new CBA is in place, what role his injury and return played in the successes and failures of the Colts passing game is an important question.
On the face of it, the results are dramatic
Peyton Manning threw 4 interceptions in the 8 games Addai played and 13 in the 8 he did not. The Colts passing offense went from 11 yards per completion to 9.9, yards per attempt from 7.3 to 6.6. The Colts had 18 more passing yards and 3 more passing TDs in 29 less attempts while Addai was playing. Overall Indy's passing offense had an adjusted yards per attempt of 7.50 and passer rating of 100.5 with Addai and an ANY/A of 5.55 with a passer rating of 84.0 without him.
Of course Dallas Clark missed every game Addai did and Austin Collie either missed entirely or left early 7 of those 8 Addai missed. The really dramatic comparison is between the Colts offense when it had Addai, Clark and Collie, to when it didn't.
In the 6 games with a healthy Addai, Clark and Collie the Colts completed 67.3% of their passes for 11.2 yards per completion and 7.5 yards per attempt. The Colts scored 13 passing TDs to just 2 interceptions, putting up an ANY/A of 7.85 and passer rating of 133.8.
Addai did return to play in two games while Collie and Clark were on IR. In those two games the Colts passing offense wasn't nearly as productive as it was with Clark and Collie healthy as well, but improved noticeably from 5.55 ANY/A and 84.0 passer rating to 6.20 ANY/A and 90.3 passer rating. The improvement coming largely from a reduced interception rate and improved yards per completion.
A small sample, but it does suggest that Addai's absence was in part responsible for the high rate of interceptions and difficulty attacking downfield the Colts had midseason. A very plausible suggestion given that Addai is a fantastic pass blocker and his primary replacement was (to put it mildly) not.
While Addai isn't the most important free agent the Colts have (obviously that's Peyton), he's very likely the most important who could actually leave once the new CBA is in place.