The Toffees are struggling in the Premier League and currently sit two points above the relegation zone.
The Toffees faithful have expressed their unhappiness with the way this season has gone, as well as the performances of former manager Rafa Benitez.
And Foster believes the home support turning on their own players is 'massive' and is something opposition teams take advantage of when playing at Goodison Park.
"If you can get on top of Everton early doors, the Everton fans are horrible to the Everton players," Foster said on his 'The Fozcast' podcast.
"They are horrible, they get right on top of them. They get in their players' heads, for sure."
When asked what it's like to be an opposition player in that scenario, Foster said: "That's massive, we always say before the game that we need to start fast, we need to get on top of them.
"We said that before our game away at Goodison Park [earlier this season], they scored after two minutes! We won 5-2, in the end.
"But, yeah, we do make a point of saying it: the Everton fans are horrible.
"If it looks like we're overrunning them or something, they get on the Everton players."