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If you were making a list of players likely to sign long-term extensions last September, cornerback David Amerson would have ranked pretty low on the list.

That would have been especially true on September 21, which was when the Redskins put the 2013 second-round pick on waivers. Amerson landed with the Raiders the next day and used the rest of the season to make the case for the four-year deal that the team made official on Tuesday.

The numbers in the deal — $17.5 million in guarantees, $5.5 million at signing — may not be as robust as initial reports indicated, but it’s still a commitment that was hard to imagine before Amerson’s strong year in pass coverage. During a conference call to discuss the signing, Amerson said the difference with the Raiders came down to an increase in his understanding of what it takes to succeed in the NFL.

“Once I got here, I matured,” Amerson said, via the San Francisco Chronicle. “I came in the league and I was 21 years old. … I was taking that transition of becoming a professional [for granted] … Compared to when I was in Washington, instead of going home and playing video games all night and stuff like that, it was a much different approach. … It’s really just the little stuff. That’s what football is; it’s a game of inches. It just gave me that extra beat that I needed on Sunday to make plays out there. Like I said, it wasn’t rocket science. I knew what I had to do and it just clicked for me at a time where, like I said, I was being released and coming here. I’m kind of glad it happened like that.”

Amerson also credited defensive coordinator Ken Norton Jr.’s scheme with making Oakland the “perfect spot” for him. The additions of cornerback Sean Smith and safeties Reggie Nelson and Karl Joseph this offseason sets the stage for the entire secondary to take the same kind of steps forward that Amerson made last year.