Theoretical warp physics -- the real kind, like Alcubierre, not the Wesley Crusher kind -- proposes that a source of negative mass would be critical to the creation of a warp field.

Antimatter as part of an energy storage system for propulsion is still a good idea. They'll just have to look elsewhere for that property, or modify the theory to work on different principles. Eric Lentz is working in that direction and doesn't think negative mass/energy is needed.