In an interview with Entertainment Weekly, the rock guitarist was asked how he would feel about a Guns N' Roses track being covered on the show.
He said: "We got asked about that once already but it got turned down.
"In the current climate of what’s going on in entertainment these days, I try to be more optimistic than negative because it’s really easy to get negative about it, but I draw the line at Glee. Glee is worse than Grease and Grease is bad enough.
"Actually, I look at Grease now and think 'between High School Musical and Glee, Grease was a brilliant work of art.'"
Slash also hinted that Velvet Revolver are close to finding a new singer.
He added: "There’s a couple of people and I can’t tell you who they are, but they’re all good. Pretty much everybody has been really, really good.
"It’s just trying to find the right fit and the right style, and somebody who just gets it."