The Raiders initially signed Marshall in the off-season after a season-ending knee injury with the Denver Broncos in 2018. He was a surprise cut at the end of training camp this year.
During training camp, the 30-year-old Marshall was spending time with the first-team defense alongside Tahir Whitehead and Vontaze Burfict. Burfict was handed a season-ending suspension by the NFL for another illegal hit earlier in the season.
Along with Burfict being out, the Raiders are without backup Marquel Lee, who is on injured reserve with an ankle injury. Undrafted rookie Justin Phillips, who suffered a knee injury in Sunday's loss to the Houston Texans, was waived to create a roster spot for Marshall.
"We're struggling at linebacker right now with Marquel Lee [hurt] and Vontaze's suspension," Raiders head coach Jon Gruden told reporters Monday. "We're going to probably be looking to shore up the linebacker position here in the next 48 hours.
"We're going to try to bring in another linebacker and train him. We have Dakota Allen, who didn't play yesterday, also on the team. We're thin. We have to add another linebacker, certainly. Hopefully Whitehead is ready to go. He came out of the game a little nicked up, too."
The Raiders will host the Detroit Lions on Sunday.