Witherspoon has played both the WILL and the MIKE position, the latter which he'll likely focus on as an Eagle at least for this year. Most reports suggest that Witherspoon is best suited to the weakside. With Omar Gaither potentially out with a foot sprain, Trotter clearly not at game speed, and Joe Mays failing to impress the Eagles obviously felt like they needed to make a move to shore up their linebacking corps.
The fear you have with any midseason trade and a reason such moves are so rare in the NFL is that it usually takes a player a long time to learn a team's system and playbook. The great thing about Witherspoon is that he's spent the entire year learning the Eagles' system from Rams' head coach Steve Spagnuolo, who spent many years here as a defensive assistant. The learning curve for Witherspoon should be minimal.
Witherspoon is signed through 2011. His current base salary is $2.5 million. Next year, it doubles to $5 million. In the final year of the deal, it remains at $5 million.