They say a defense prepares for an opponent's offensive system, in which case Sean McDermott's group is readying for a steady diet of running back Chris Johnson to lead the way for Tennessee's offense on Sunday. But it is impossible, with kickoff less than three days away, to know who is starting at quarterback for the Titans, and that presents a definite challenge to the Eagles defense.

Do the Eagles have to include a "contain" element in their scheme to cage in the mobility of Vince Young, who is a dangerous runner in the option and when the pocket collapses. He is a big quarterback, and a very fast one. He is strong and able to break tackles and he has just enough wiggle in his running style to make defenses miss.

But Young hasn't practiced all week after "tweaking" his knee and ankle in Monday night's win over Jacksonville. Kerry Collins, a sturdy veteran with whom the Eagles are very familiar from his days as a New York Giant (1999-2003), is in line to start on Sunday, unless all of that changes in the next 48 hours and Young practices and, well, this sounds like the perfect "game-day decision."

Except that maybe there isn't anything "game day" about this decision. Maybe Tennessee head coach Jeff Fisher is keeping alive the notion that Young can play just to mess with the Eagles' heads.

In any event, the defense is in the rare position of not knowing which quarterback will start, and while normally that isn't a huge problem, in this case the styles of Young and of Collins are so different that there have to be some significant differences in the way the Eagles approach some situations against the Tennessee quarterback.

McDermott will have the defense ready, no matter who plays quarterback. The defense has a lot of confidence -- a lot of work to do to be where it wants to be, but a lot of confidence, nonetheless -- these days and they are well aware that they face the premier running back in the game, Chris Johnson, an excellent offensive line, a big and strong and physical receiving corps and a quarterback -- both Young and Collins -- who runs the offense well and with excellent efficiency.

This is a game for the defense. For the Eagles to win on the road, in a hostile environment, against an outstanding team, the defense has to rise to the challenge.

* NEWS, NOTES AND A LITTLE BIT OF THIS AND THAT*
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 I'm glad that Bobby April named Jorrick Calvin the return man on punts and kickoffs. Calvin has displayed good hands, fine decision-making skills and a great burst in the return game. The special teams have a huge task on Sunday. Tennessee's return game has been highly productive this season. Is this the game that April's group wins for the Eagles?  

• Can the Eagles run the football against the Titans? Well, the Eagles seem as committed to the running game as they have been in a long, long time with LeSean McCoy leading the way. The Titans are excellent against the run, though, and as my buddy Adam Caplan from FoxSports.com points out, Denver beat the Titans in Tennessee by throwing the ball. A lot.

• No word on Jerome Harrison's potential role in the offense. He says he is ready, if called upon. But it may be too early to expect much from Harrison, who is still learning the offense.

• The test for King Dunlap is every bit as enormous as it was last week when he played against John Abraham. The Titans have studied Dunlap for a full week. They are going to try to expose his weaknesses. They saw how the blocking scheme protected Dunlap at times last week. So, the Eagles have to be ready for the Titans to overload Dunlap's side, and to use the attention the Eagles are paying to Dunlap to take advantage and apply pressure in other areas on the line. It is a critical chess match, of course.

• Can't wait to see the thousands of Eagles fans at LG Field on Sunday. This is a big travel game for the fans, and, as always, we appreciate the support and it definitely means something on the road. The Eagles are 3-0 away from Lincoln Financial Field this season.

• Will there be a young group of wide receivers on the field for the Eagles than for any team in the league on Sunday? Jeremy Maclin is in his second year and both Riley Cooper and Chad Hall are rookies. Jason Avant, the old man, in his in fifth NFL season. Pretty remarkable. We'll learn a lot about Cooper and Hall in this game. Both are going to have to play and both are going to have to produce. They're being asked to grow up very quickly.

• Another aspect of the game to watch: The Eagles have to match how physical the Titans play. Tennessee might be the most technically-sound and physical team the Eagles play all season.