Diggs had been a starter for the Lions since 2017 at slot cornerback and safety. Over the past two seasons he had become one of Detroit's most reliable defenders in the secondary, along with Pro Bowl cornerback Darius Slay.
Diggs, 26, was a Pro Bowl alternate last year after a 78-tackle, three-interception season with eight passes defended. This season, he missed one game and part of another due to a hamstring injury, but had been on the field for a high percentage of snaps otherwise.
He wasn't playing as well as he did in 2018, but still had 20 tackles through five games for the Lions.
The Seahawks already have a logjam at safety, although starter Bradley McDougald is dealing with back spasms that kept him out Sunday against Baltimore and Lano Hill is expected to miss at least another week with an elbow injury. Rookie second-round pick Marquise Blair impressed coach Pete Carroll on Sunday while making his first career start. Free safety Tedric Thompson, meanwhile, allowed a 50-yard completion on Baltimore's first possession that Carroll lamented as a "big mistake."
"I hate that we gave them that," Carroll said. "There's no way that should ever happen."
Trading Diggs means the Lions are likely to now start third-round pick Will Harris at safety along with Tracy Walker, the team's third-round pick a season ago who replaced the retired Glover Quin in the lineup this season.
Detroit does have an abundance of safeties they can use with Walker. Besides Harris, both veteran Tavon Wilson and rookie C.J. Moore could see more work in the defensive backfield, particularly in nickel and dime packages.
But moving on from Diggs is the second straight season Detroit traded a player popular in the locker room and with fans. Slay was among Lions players to react on Twitter, calling the trade "bulls---" in one of his tweets.
This one hit me bra😢 https://t.co/coKjDwUrTq— Darius Slay (@_bigplayslay23) October 22, 2019
Nooooooooooooooooo— Mal (@jamalagnew) October 22, 2019
Wowwwwwwwww no way— Damon Harrison Sr. (@snacks) October 22, 2019
Last year at the trade deadline, the Lions traded Golden Tate to Philadelphia for a third-round pick. Tate returns to Detroit on Sunday to play the Lions with his new team, the Giants.
Unlike the Tate trade, Diggs was not in the final year of his contract. Diggs signed a three-year, $20.4 million extension with Detroit last season. He is under contract until 2021 and has cap hits of $6,991,666 in 2020 and $7,341,668 in 2021 but no guaranteed money in either season.
ESPN's Brady Henderson contributed to this report.