It's all part of the "tank". The week before the Matthews trade, Matthews was looking like he lost his job in Philly. He was on his way out. Bean and McD got together, agreed to tank, but McD's reputation is built on Defense. He likely ensured the Defense looked good so his reputation was still solid. Think about it. McD has been running good Defenses under Defensive minded HCs or an assistant D Coach. In Philly, he was under Jim Johnson as the assistant. After McD replaced Johnson, he only lasted two years until Ried, uncharacteristically, fired McD. Then he goes to Carolina under Defense HC Ron RIvera. So far, McD has only been good under Defensive minded HCs. So if this whole HC thing goes south, he has to ensure he will be hired again as a DC somewhere. So far he is doing a great job with the Defense, but so was Schwartz with similar players. Rex just jacked it all up. Matthews will be a FA next year and will likely head somewhere else. The Offense will be in complete rebuild next year, with a new QB, new WRs and maybe even a new RB.
http://www.csnphilly.com/the700level/speculation-about-jordan-matthews-future-eagles-ramps-again
http://www.nfl.com/news/story/09000d5d81db2c4c/article/eagles-fire-dc-mcdermott-who-could-land-in-denver-carolina
http://profootballtalk.nbcsports.com/2011/01/17/sean-mcdermott-lands-on-his-feet-in-carolina/