Here are possibilities on how Shonn Greene's 2009 season could play out depending on circumstances.
If Thomas Jones is healthy and productive: Greene's opportunities are limited. He sits and learns from a back with a similar style. He has time to develop at his own pace. The learning curve is not steep at the position, but the year gives him a chance to pick up some of the nuiances of the pro game like blocking through reps. in practice. Jones is cut after the season to save cap space, leaving Greene as the starter in 2010.
If Thomas Jones is traded for receiver help (like Braylon Edwards): Greene steps in as the go to back. The move adds a lot of pressure, but it also takes some off. With an elite receiver to stretch the field, the team can rely a bit more on the passing game and will not face as many eight man fronts.
If Thomas Jones gets injured: Greene has likely had at least some time to adjust to the pro game and lifestyle. The team did not want to demand so much of him so soon, but he needs to step up and deliver what Jones did in the offense. To the surprise of many, the star rookie from the Big Ten shows the ability to do just that right off the bat.
If Thomas Jones finally hits the wall: The new coach left Jones with the starting job because of his 2008 campaign but has no particular loyalty to a player whose first act under this new regime was to skip workouts and demand a new contract. Greene gets tabbed halfway through the season and never gives the starting job back.
If the Jets cannot find even adequate quarterback play: Greene takes on a bigger role than expected as the Jets are forced to rely on the ground game even more than they expected. Greene sees a load of carries and serves as earth to Leon Washington's wind and Thomas Jones' fire in a three back rotation.