It eventually worked, too. San Francisco coalesced into one of the league's top defenses last year, with its front four leading the way to a Super Bowl run.
But with great success comes great financial strain.
Armstead, coming off a career year, was set to be an unrestricted free agent in March. With Buckner a year out from free agency and likely staring down a big contract himself, the 49ers front office seemingly faced a fork in the road between the two.
"I didn't have any knowledge of it, so I signed and then I heard of that, so it was a shock to me," Armstead said on KNBR 680's Papa and Lund show, per NBC Sports.
Their shared history probably made the trade more poignant. The two had been teammates for seven of the previous eight years, dating back to their time at the University of Oregon. Buckner developed a bit quicker out of college, contributing more in their respective first two seasons, earning a Pro Bowl bid in 2018 and following it up with another strong performance in Year 4.
That consistency coupled with a year left on his contract made Buckner a major asset. The 49ers ultimately decided the pair would be too expensive to keep and that having Armstead and the No. 13 pick in the 2020 draft was more valuable than Buckner alone.
"It was a shock. You know, I was confused and I didn't really expect that," Armstead said. "Super excited and happy for him. Me and him actually have been working out for a few weeks now, so we've been able to hang out and talk about it. God works in mysterious ways and we both believe things happen for a reason. It's going to be weird not playing alongside each other anymore like we have done for about eight years, but I know he's going to do amazing things up there with the Colts. I'm super happy for him."