He must let Rider know of the situation here. Waver, after deciding as such, began to concentrate his thoughts to calling out his own Servant. Having no knowledge of telepathy, Waver could only rely on speech to come to a mutual understanding. However, Rider, who knew this, did say that ‘I’ll leave a herald for you”.

The space beside Waver abruptly shook, and the form of a knight emerged.

“Mithrenes of the Hetairoi rides forth to listen in the King’s stead!”

Overawed by the intrepid demeanor of the Heroic Spirit and his simple salutation, Waver faltered. However, he changed his mind, knowing that it was not the time to pay attention to such things, and mustered the courage in his heart to give directions to this Heroic Spirit he’s never met before.

“I want you to release the bounded field and throw Caster out to the appointed location as soon I give the signal. You can do that, right?”

“It can be done – but it is a race against time. It appears that our army inside the bounded field cannot stop that sea demon for much longer…”

“I know! I’m aware of that!”

- Fate/Zero: Act 10, Part 5