Those tuning into Saturday's Alamo Bowl who saw TCU stage a 31-point comeback over Oregon weren't wrong in calling the rally one of the most unlikely in bowl history.



Yet it should not have been a shocker for anyone who'd followed Oregon's 2015 season.



In many ways for Oregon it was the purest, if painful, distillation of a 9-4 season spent on the brink, where production moved between boom or bust and six games were decided in the final minutes, if not the final series.



"They made one more play than we did," coach Mark Helfrich said.



The Ducks walked a tight rope all fall. At their best, they appeared the soaring program that has so often traded with Stanford Pac-12 Conference bragging rights, deserving of their No. 7 preseason AP ranking and a handful of plays away from finishing the regular season with one loss. Just 3-3 on Oct. 10 and out of the polls, the Ducks won six consecutive games, including against Stanford and USC, the conference's North and South division champions. Given all that went wrong at the start, and without the aid of a veteran defense and a Heisman Trophy-winning quarterback, a case could be made that that stretch, which propelled UO toward the Pac-12's No. 2 finish, was Helfrich's best coaching job of his short tenure.



But at worst, they proved to be one false move away from a steep drop, with little safety net in the way of depth. This was clear in a 62-20 home loss to Utah, a blown 10-point lead in the fourth quarter against Washington State, the meltdown against TCU and in curious play calling against Arizona State and late against TCU.



The season's anxious finish also was a hallmark of its hand-wringing start: with all eyes on quarterback Vernon Adams Jr.



After announcing he'd transfer from Eastern Washington in February, Adams endured an uneasy six months until finally joining the roster in mid-August, four days into fall camp, after he passed the one final math test he needed for admission. Adams wasn't perfect once in uniform and the warts from learning an offense on the fly showed, but he finished as the FBS leader in passing efficiency (179.1) and arguably its best deep-ball thrower, too.



Once sophomore Darren Carrington returned from a half-season suspension Oct. 17 at Washington, the uncanny connection between the quarterback and receiver that was forged during unofficial offseason workouts was revealed to opposing secondaries caught unaware. Carrington averaged 19.0 yards per catch and hauled in a touchdown once every five receptions. Along the way, two injured Ducks who sat out the 2014 season returned to their places as foundational pieces. Left tackle Tyler Johnstone earned All-America honors, and Bralon Addison gained 1,000 all-purpose yards as a receiver and returner and even threw a touchdown against Colorado in one of the season's most memorable plays.



The bigger test for Oregon, however, was what it would do when Adams could not play, such as when he broke a finger in the season opener and missed parts of four games, or when he left the Alamo Bowl in the second quarter after a hard hit to the helmet. Oregon gained zero yards in the third quarter against TCU, and just 43 after halftime on 31 plays.



For those on the outside looking in at UO, the math didn't add up: five QBs on the roster, but only one who consistently made the offense go.



UO had few answers as it turned to veteran reserves Jeff Lockie (QB rating: 119.4) and Taylor Alie (124.0) for help.



"Points were at a premium for us in the second half," Helfrich said after the loss to TCU, in which the Horned Frogs played without a pair of All-Americans in quarterback Trevone Boykin and receiver Josh Doctson. "We'll take that as a tough pill. ... If they go in there, we expect them to play great. It's that simple."



Along the way, the defense bent -- and bent some more, in allowing a school-record 37.5 points per game -- while producing late-game stands to preserve wins against Washington, Arizona State, California, Stanford and USC. On his way to becoming a likely high first-round NFL draft choice this spring, defensive end DeForest Buckner won Pac-12 defensive player of the year honors.

Oregon's special teams personified the season's extremes, too. Sophomore kicker Aidan Schneider made 91.7 percent of his field goals, second-best in the country, and UO scored twice on returns. Yet a fumble by the normally electric Charles Nelson allowed TCU prime scoring position that aided the comeback. Jarringly, opponents blocked three Oregon punts and ran back three others for touchdowns -- this from a unit that, entering the season, hadn't given up a punt-return TD since 2007.



Nearly lost in the shuffle and a strong national class of running backs was the prodigious production of sophomore running back Royce Freeman. His 1,836 rushing yards are a new single-season school record, while his 2,184 all-purpose yards rank No. 3 all-time at UO, behind Jonathan Stewart in 2007 and De'Anthony Thomas in 2011. The Ducks finished fifth nationally with 43.0 points per game, and averaged 39.7 against Power Five opponents, though each was aided by two triple-overtime games.



Adams' brilliance at extending plays only made the void when he couldn't suit up appear even larger, however, which underscored Oregon's lack of options roster-wide at critical spots such as center and defensive back. Oregon's "next man up" mantra rang hollow at times, such as when center Matt Hegarty was injured in the Alamo Bowl, and replaced by Doug Brenner, whose persistent -- and as he said, uncharacteristic -- low snaps forced Lockie to hesitate and allowed more time for TCU to apply pressure.





Depth issues hamstrung UO's ability to grind down opponents as it did in recent seasons, when its speed and depth arrived in waves. The defense, in particular, didn't have such a luxury. The secondary began with the daunting prospect of replacing three departed senior starters with underclassmen, but within the first weeks Chris Seisay, the only returning starter, was hurt, and UO patched one hole at safety by plugging in Charles Nelson, a wide receiver and returner who'd practiced at corner in spring.



A Catch-22 emerged: Should the Ducks blitz but leave defensive backs on their own, or allow quarterbacks more time to throw by devoting more defenders to coverage? In the Alamo Bowl, UO chose the former in the first half, then backed off the blitz in the second. Oregon's passing efficiency defense ranked 96th.



"The thing we have to do here at Oregon defensively is to establish more depth so that when games start to wear like this where we can rotate more guys like we have in the past and still maintain a certain level of play," said Don Pellum after the Alamo, two days before he was demoted from defensive coordinator to linebackers coach.



Appearing gassed, Oregon couldn't keep up against Utah and couldn't close out late leads against Washington State and TCU. In the Civil War, it barely held onto its three-touchdown halftime lead before eking out a 52-42 home win against rebuilding Oregon State. Opponents outscored Oregon 317-257 after halftime.





"This whole year, especially on the defensive side, it's been a struggle in the second half to finish games," Buckner said. "Just like the Oregon State game ...We gave all of that up."



TCU's comeback was an echo of the Civil War and tagged Helfrich with his first loss, in 29 games, when UO scored more than 40 points. On Monday, Pellum was demoted to linebackers coach after two seasons calling the defensive plays.



In a program that prizes continuity, new voices could be joining UO's staff soon. Clancy Pendergast, currently with the San Francisco 49ers and reportedly a candidate when UO's coordinator job opened in 2013 -- he and Helfrich share an agent -- is said to be a candidate again. Whoever is hired will make it two new coordinators for Oregon in 2016, after Matt Lubick was promoted to offensive play caller the day before the Alamo Bowl.



Two days before the finale, Pellum said it best about the game, and season, that never let Ducks fans get comfortable.



"We have to be prepared," he said, "for everything."



-- Andrew Greif

agreif@oregonian.com

@andrewgreif