David Spade -- Wes Welker's Killin' My Fantasy Team

Concussions + molly (allegedly) + Emmanuel Sanders = a TERRIBLE fantasy football season for Wes Welker ... and David Spade knows it.

The star of the upcoming "Joe Dirt 2" (yes!) was leaving the Mercer Hotel in NYC this weekend when we asked who his biggest fantasy bust has been this season.

That's when Spade -- who also hosts "Fantasy Football Uncensored" on Fox Sports 1 -- explained why he thinks Welker's been crappin' the bed ... and it's pretty hilarious.