Hagar Stone and Discards

How does Hagar Stone interact with cards that require you to discard cards from your hand?
Do you replace them?

If you do replace them, I supposed it's not an issue with cards like The Barrens, but it's a book keeping nightmare for cards that require you to discard your hand (e.g. Legion)

Name: Hagar Stone
Capacity: 6
Type: Vampire
Group: 5
Clan: Malkavian antitribu
Disciplines: obf AUS DEM
Set/Rarity: SoC:V
Artist: Ken Meyer, Jr.
CardText: Sabbat. Black Hand: While Hagar is ready, you have +1 hand size. While Hagar is untapped, do not replace cards you play on your turn until Hagar taps or your turn ends.