And for the next three or four years, California will wonder what might have been.
"It came down to Cal and Oregon," Armstead's father Guss said.
The Golden Bears were so close to landing the No. 2 overall player in the Class of 2012 that the tipping point may well have been the loss of assistant coach Tosh Lupoi to Washington weeks before signing day.
Arik said the shocking defection of Lupoi "definitely was a big factor.
"The dynamics of the situation changed and things changed. It definitely affected the situation," he said.
For months, Cal seemed to be the obvious frontrunner for the five-star defensive linemen.
It was close to home. "An hour and 15 (minutes away), my parents can watch me play all the time," Armstead said.
It was to play for programs with which he was intimately familiar, relaying how his first recruiting letter came from Cal basketball. "They spelled my name wrong. Eric. I always remember that," he said.
It was a melding of academic and athletic opportunity. "I feel there is a need for me in both football and basketball. Also it is very good academically, one of the top public schools in the country. A good fit," he said.
Was losing Lupoi enough to sway his decision? Who can say, but it certainly made a difference.
What is clear is that it could set off a chain of events that could devastate Cal for years to come.
With the arrival of Mike Leach and Rich Rodriguez, the always offensively driven Pac-12 has become even more spread out. The only way to neutralize such attacks is with superior defensive line play.
While Cal has recruited very well in that regard, Armstead's physical gifts are well suited for a 3-4 defensive lineman. He can drop into coverage with his nimble footwork, but also occupy blockers and generate pressure.
Those skills will only solidify an Oregon defense perfectly built to compliment the Ducks' up-tempo spread option attack.
It also could sway other top Cal targets.
Armstead said he would like to play with Sacramento (Calif.) Grant safety Shaq Thompson.
"I talked to Shaq today and he is going through his own process," he said. "We would love to play together but if it doesn't work out, we're always going to be boys. Hopefully it works out for him to come to Oregon with me."
Thompson, a five-star prospect in his own right, offers the kind of skill set equally necessary to defend the proliferation of spread offenses on the back end. Safeties who can really deliver in pass coverage will be just as valuable as an elite lineman like Armstead.
It would now appear to set up as a three-way race between Cal, Oregon and Washington, where he took an unofficial visit this weekend, for Thompson. To miss out on him and Armstead in the same class when weeks ago they were expected to lead a breakthrough group would be a stunning setback.
With a recruiting dead period now in effect, there is nothing left for coach Jeff Tedford and his staff to do between now and Wednesday but wait and hope Armstead is the only player they lament having to line up against for the next three or four years.