Biggest shoes to fill: USC

Starters in, starters out. That's college football. Players' eligibility expires, and they leave for the rest of their lives, whether that includes the NFL or not.

And they leave behind shoes of various sizes that need to be filled.

Our concern with this series? The biggest shoes -- in some cases Shaq-like size 23s.

Big shoes: Offensive tackle Matt Kalil

Big shoes? How about this: Kalil is almost certain to be the first offensive lineman picked in the NFL draft on April 26. And he's almost certain to go among the top-five picks. So these are very big shoes, in addition to the 6-foot-7, 295 pounder being one of only five 2011 starters not returning this fall. Further, Kalil protected quarterback Matt Barkley's back side, and did it very well, not yielding a sack. In fact, he was the cornerstone of a line that surrendered just eight sacks, second fewest in the nation.

Stepping in: Sophomore Aundrey Walker or junior Kevin Graf

Walker might have pulled ahead of Graf, who started at right tackle in 2011. The 6-6, 320-pounder was listed at 375 pounds when he played guard as a true freshman a year ago, but his athleticism has long been obvious. If he continues in his present trajectory of conditioning and performance, he'll be the next Trojans offensive lineman to get picked in the top-half of the first round of the draft. Whoever replaces Kalil, the other will almost certain start at right tackle. Both will be charged with protecting USC's most precious asset, the nation's leading Heisman Trophy candidate.

You can check out the rest of the "Big Shoes" series here.