She could easily win with Excalibur, but that would reveal who she is to everyone. Lancer and Lancer's master now know who she is, but right now, no one else does. Lancer was the only one close enough to recognize Caliburn through Saber's Invisible Air. But we should expect that more than just Rider and Assassin were watching.

Knowing a Servant's identity changes the entire battle. Saber would not have even gotten hurt if she had known who Lancer was from the start.