Ok Davis has said he will not play again until his knee is 100%. He has bone on bone contact, he refuses to go under the knive again, and he is what he is. A RB with bad wheels.
Also the team giving him the year on the IR changes how the team gets to deal with the cap. They would have had to give him an injury settlement to cut him, which would have amounted to close to the salary he would be getting this year. By putting him on the IR they get a roster space, they force Davis to sit and rest the leg.
Come spring time, when he starts to run again, either he will have missed the game enough to play with the pain that will always be there, or he will call it quits.
By having DD retire, the team saves the injury settlement, and the team can ask for some of the bonus money back, (probably will not be done). And they bought another year to spread the Bonus Money on the cap to. Smart move business wise, so this could not have been thought up by Casserly, besides he was gone before training camp when this all became very clear there was a bigger problem.