L1 manually switches players on defense.
However I don't see why you're having the problem of the game auto-switching away from the player you're wanting to use for a tackle (even a slide tackle)?
As you point out, the game highlights your player in closest proximity to the opposition player with the ball. Wouldn't that be the player you'd be using for a slide tackle?
However, if for some reason you wish to switch to a player that isn't in closest proximity to the opponent with the ball, then simply click on L1.
I know this is a late post, but hopefully it will be of assistance to someone.