Terrell Davis tells TMZ Sports Sean Payton has made such a huge impact since taking over the Denver reins this offseason ... it actually reminds him of the way Mike Shanahan turned around the team in the 1990s.

"It's different already," the former Broncos tailback said. "In terms of just the energy and the vibe around the building."

"This reminds me of Mike Shanahan when he first came to Denver," added Davis, who played for the legendary Denver coach from 1995 through 2001, winning two Super Bowls. "A lot of things that Mike did, it sounds like Sean Payton is the same type of coach."

One of the biggest positive changes Davis has noticed already with the team is the workman-like mentality the players have adopted.

He tells us he's not seeing nearly as much activity on social media from the guys  -- and it appears everyone is far more focused on winning games than they were last year, when they piled up an abysmal 5-12 record.

"They're just working," Davis said. "He's changing the culture there. He's putting his own stamp on things behind the scenes."

Davis, though, tells us he's not necessarily expecting immediate results in Year One ... explaining he's anticipating some growing pains in Payton's first season with a new franchise.

But, he is predicting a huge jump for Russell Wilson nonetheless ... telling us "this is exactly what Russ needs."