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1. Chip Kelly - Coach - Oregon
Why so hard to fill? No coach in conference has ever been as successful and as efficient as Kelly was during his time at Oregon. It's not just Kelly's winning that will be so hard to replicate either, as he was a freakishly prepared and detail-oriented coach who created and crafted the Ducks' nearly unstoppable system himself so he knows it better than anyone else will.
Who has to fill them? As the Ducks' offensive coordinator the past four seasons, there was no smoother option to take Kelly's position than Mark Helfrich. Helfrich has no head coaching experience, but was long groomed as Kelly's protégé and knows the system probably just as well as anyone other than Kelly.
2. Johnathan Franklin - RB - UCLA
Why so hard to fill? Franklin led the Bruins in rushing the past four seasons and finished as the Bruins' all-time leading rusher. Not only was he productive on the ground, but he was also a sturdy back that could shoulder a ton of carries while also catching the ball out of the backfield.
Who has to fill them? It's not really clear yet as no one established themselves during Spring football, but expect senior Damien Thigpen and junior Jordon James, their leading returning rushers from 2012, to carry most of the load. The more important question though, is whether or not they can match Franklin's production which doesn't seem likely.
3. Matt Scott - QB - Arizona
Why so hard to fill? Scott may have been the most individually important player to any Pac-12 team last season as he was absolutely integral to the Wildcats' up-tempo offense executing as well as it did. A perfect balance of accuracy, speed, athleticism, experience and leadership, Scott was the ideal kind of quarterback that Rich Rodriguez needed to get his system running smoothly from day one.
Who has to fill them? Senior B.J. Denker stepped in for Scott when he was hurt against Colorado and excelled and it looks like he has an initial, slight lead over junior Jesse Scroggins for the starting position now. Yes, Denker looked good against Colorado, but almost any quarterback could have last season and Denker doesn't have the experience or physical tools that Scott did going into last season.
4. Matt Barkley - QB - USC
Why so hard to fill? Barkley has started for the Trojans for the past four seasons and was seen as the Trojans' leader and heart and the soul for the past couple of them. When on, there was no quarterback in the Pac-12 as dangerous with the ball in his hand as Barkley.
Who has to fill them? Sophomores Cody Kessler, Max Wittek or freshman Max Browne will assume the starting role sometime in the Fall, but all other than Wittek are very inexperienced and none has generated much excitement.
5. Stepfan Taylor - RB - Stanford
Why so hard to fill? Taylor ran for more than 1,000 yards each of the past three seasons and was the Cardinal's battering ram that carried the ball the most in their smashmouth offense. Taylor carried the ball more than just about anyone in the conference and his toughness, durability and endurance might be what is the hardest to replace.
Who has the fill them? Seniors Anthony Wilkerson and Tyler Gaffney have been solid fixtures in the Stanford attack for years, and both are big backs that are similar to Taylor but have yet to have been counted on to take on a Taylor-level workload along with expectations.
6. Zach Ertz - TE - Stanford
Why so hard to fill? Ertz had the best stats of any major tight end in college football last season and was pretty much the Cardinal's only consistent and dangerous receiving threat. The Cardinal also lost their number two tight end, Levine Toilolo to the NFL, so there is an especially big hole.
Who has to fill them? It sounds like 6'7 260 sophomore Luke Kaumatule has the physical tools to try and fill Ertz' shoes and started to establish himself as the starter during the spring but there isn't much else there.
7. Dion Jordan - DE - Oregon
Why so hard to fill? Always hard to fill a guy who goes third in the draft and Jordan filled a very valuable role as the Ducks' primary pass-rusher.
Who has to fill them? It might surprise those that don't follow the Ducks closely that returning senior defensive end Taylor Hart actually had three more sacks than Jordan last season, but that is precisely why Jordan isn't higher on this list. However, it is also yet to be seen how Hart will do without Jordan drawing a great deal of focus on the other side.
8. Kenjon Barner - RB - Oregon
Why so hard to fill? With more than 1,700 yards and 21 touchdowns, Barner stepped up big time in 2012 and completely filled LaMichael James' huge shoes and while Oregon's offense has seemed to seamlessly transition from running back to running back, there is no proven commodity behind Barner. While De'Anthony Thomas is the most dangerous playmaker in the conference, it's unclear if he can, or if the Ducks really even want to use him as an every down style back like Barner was.
Who has to fill them? Obviously Thomas is going to be expected to step up his production on the ground, but the more important need to fill here is the between-the-tackles, every down running and that will likely be pushed towards Byron Marshall and possibly true freshman Thomas Tyner. Both Marshall and Tyner are bigger runners who fit the mold of backs that can handle a lot of carries.
9. Austin Hill - WR - Arizona
Why so hard to fill? Most might not realize it, but Hill was the Pac-12's leading receiver after Marqise Lee in 2012 so it was a devastating blow for the Wildcats when he was knocked out for the season with a torn ACL in Spring.
Who has to fill them? Star running back Ka'Deem Carey will probably be used as a pass-catcher even more but the rest of the expectations will fall on lightly proven returning receivers David Roberts, Garic Wharton and Tyler Slavin.
10. T.J. McDonald - S - USC
Why so hard to fill? McDonald was one of the best safeties in the country and a guy that definitely hurts to lose, but a lot of problem here is that the Trojans simply don't have anyone who looks like he can replace him right now.
Who has to fill them? Junior Josh Shaw saw some good time at corner in 2012 and moved to safety where he is expected to start. Senior Demetrius Wright is another lightly-experienced safety that will compete for a starting spot along with highly-touted freshmen Su'a Cravens and Leon McQuay III.
11. Robert Woods - WR - USC
Why so hard to fill? Obviously the Trojans are fine at wide receiver with Marqise Lee, but you have to wonder how much of Lee's ridiculous productivity had to do with the fact that defenses also had to worry about Woods. Woods kind of got upstaged by Lee, but he was arguably the second-best receiver in the conference last season.
Who has to fill them? Sophomore Nelson Agholar came on in spurts towards the end of the season so he will likely be expected to become the number two receiver across from Lee but also keep an eye out on true freshman Steven Mitchell who was maybe the top-rated West Coast receiver in the Class of 2013.
- Star Lotulelei - DT - Utah
Why so hard to fill? Lotulelei was one of the most talented defensive tackles to ever play in the conference and he pretty much completely shut down the entire middle of the defense for the Utes the past two seasons.
Who has to fill them? After Spring practice it looks like it is going to be an effort by committee. Unproven seniors Tenny Palepoi and LT Tuipulotu and junior Sese Ianu.
Other big shoes to fill
Jordan Poyer CB Oregon State
Desmond Trufant CB Washington
Kiko Alonso LB Oregon
Michael Clay LB Oregon
Chase Thomas LB Stanford
Joseph Fauria TE UCLA
Datone Jones DE UCLA
Nickell Robey CB USC
Keenan Allen WR Cal