1. Vernon Adams QB Oregon - Crazy that a player who was only at his program for a few months is the hardest to replace player, but there is no denying no player in the conference's presence played a bigger role in the overall performance of his team. The Ducks looked like a national title contender with Adams firing on all cylinders but looked like a four-five-win team without him. The Ducks are trying to replace him with another Big Sky grad transfer, but it's almost impossible to think he will be anywhere near the player Adams was.
2. Jared Goff QB Cal - Never easy to replace a three-year starter at quarterback who probably goes in the top five of the NFL Draft (just ask Oregon). Goff has been the face of Cal and their Bear Raid for three years now and had more pressure on him in 2015 than any other player in the conference. His accuracy and ability to hang in the pocket and deliver lasers on third down will be impossible for Cal to replace in one year.
3. DeForest Buckner DE Oregon - The Pac-12 Defensive Player of the Year at times seemed like he was carrying the entire Oregon defense on his back. It is scary to think about how bad the porous Duck defense may have been had he not stayed for his senior season and we might find out next year who bad that could have been as he heads to the NFL as a potential top five pick.
4. Kevin Hogan QB Stanford - Stanford finds may have thought they would be happy to see Hogan go after disappointing 2013 and 14 seasons, but that is no longer true after his stellar 2015 campaign. He wasn't electric in the way Adams and Goff were, but he put it all together to be an almost ideal quarterback for the Stanford offense as a senior. The Cardinal have some blue chippers ready to replace him, but it is unlikely they will be able to match his mastery of the Stanford system in their first year.
5. Devontae Booker RB Utah - Booker has seemingly been the only good skill position player on Utah for the past couple of years. His worth was shown in 2015 when the Utah offense really struggled once he went out with an injury. Joe Williams showed the ability to be reliable when filling in for Booker in 2015, but he is going to have to really step it up to fill his shoes.
6. Paul Perkins RB UCLA - Perkins has been a rock for the Bruin offense and one of the nation's most underrated players for years. He has racked up countless yards and was always reliable when the Bruin passing game sputtered. The Bruins have blue chip backs waiting in the wings, but they have to prove they can be the consistent force Perkins was.
7. Cody Kessler QB USC - Kessler isn't very celebrated for the numbers he put up in his career because he didn't win a ton of games. I don't think he is an irreplaceable player, but a big part of the problem is USC doesn't appear to have any slam dunk prospect waiting in the wings. Junior Max Browne is a former five-star recruit, but the fact he didn't appear to every really challenge for the job before four years into the program raises a lot of concern.
8. Joe Dahl T Washington State - Luke Falk might be the star of the 2015 Cougar uprising, but Dahl was the unsung hero. One of the best pass blockers in the nation, Dahl's importance was seen when he missed the Apple Cup and the Cougars really struggled with pass protection. One of the biggest issues with losing Dahl is the Cougars are not well-stocked with proven offensive tackles who can step right in.
9. Destiny Vaeao DE Washington State - Speaking of Cougar unsung heroes from 2015, Vaeao was a monster up front in 2015 and the biggest reason the Cougar defense improved by leaps and bounds. His presence in shutting down the run and getting after the passer will be very hard to replace. The Cougars have nice young players in Daniel Ekuale and Hercules Mata'afa, but it's unlikely they will be able to fill his huge void right away.
10. Antonio Longino LB Arizona State - The Pac-12's best pure pass rusher, Longino was unstoppable on the edge when he was on. Todd Graham's defense relies on excellent pass rushers and Longino was the perfect fit as a hybrid who could get into the backfield and finish the job with a sack or a tackle for-loss.Graham has a lot more talented linebackers, but needs one to step up and be as dynamic as Longino next year for his defense to improve.
11. Joshua Garnett G Stanford - The Outland Trophy winner turned into the All-American lineman that makes Stanford's systematic offense deadly. It took him four years to fulfill his huge potential and he is already gone. Replacing Garnett will be especially tough after center Graham Shuler retired a year early and without as many blue chip linemen waiting in the wings as their used to be in Palo Alto.
12. Kyle Murphy T Stanford - The other major stud Stanford offensive lineman who is now off to the NFL. He wasn't as celebrated as Garnett in 2015, but he was just about as good and played a more important position on the line. Same with Garnett, the trouble replacing Murphy might be that Stanford hasn't cleaned up with blue chip offensive line recruits as well as they did in Garnett/Murphy's class.