This is a preview -- and only a preview, mind you -- of USC's nightmare scenario.

The Trojans are so solid in their starting 22, but so questionable in their depth that one major injury to a load-bearing player could cause the whole thing to collapse.

Linebacker-turned-running back Tre Madden isn't that load-bearing player. But he certainly was expected to take some of the pressure off of Curtis McNeal. News broke earlier today that Madden will miss the entire 2012 season with a torn ligament in his left knee -- a non-contact injury that occurred when he planted off his left leg.

Writes Erik McKinney of WeAreSC.com:

Through only a handful of practices after making the transition from outside linebacker, Madden had impressed with his combination of balance and forward lean and looked like he would make a case to become the big back the Trojans could rely on this season. With Madden out of the lineup, the Trojans are again down to three scholarship tailbacks, as redshirt senior Curtis McNeal will be relied on heavily to carry the load this year.

Madden, a 6-0, 220-pound sophomore from Aliso Vieja, Calif., was the talk of USC's spring ball for both his versatility to make the position change and his athleticism. As a true freshman, he appeared in all 12 games last season at linebacker and special teams, notching 15 tackles and one for a loss. He started one game at Colorado and had a career high three tackles against Washington.

This kind of scenario was always the elephant in the room for the Trojans -- whether they could last an entire season without having to dip too far into the depth chart. Madden's injury leaves the Trojans right back where they were before spring in terms of running backs. Only now, they are also down a linebacker.