The Red Devils are set to welcome new boss Ruben Amorim when the international break starts.
The Sporting CP boss will bring in tactical changes, with Butt believing that he could also usher several players out the exit door.
He said on talkSPORT's White and Jordan: "Regardless of the decade we're in with Man United, I think every player should be under the impression that someone can come and take you place at any time.
"I think that's what big football clubs are like. There are probably two or three players in every generation that know they're okay - a Roy Keane, an Eric Cantona.
"But the rest of the players must always be looking over their shoulder thinking at any one point, there's a young player coming through or the manager is going to buy a world star.
"We have to get back to that at Man United.
"For me, everyone should be nervous about what's going to happen over the next 9-12 months."
Butt continued: "The players aren't standing out, they aren't playing at the highest level.
"You saw it at the end of last season, how many times are Man United players getting near the Team of the Year?
"There's none, it's sad to say but that's a fact."