Sub for ejected player

[LDUB]

I was watching the Cubs/Cardinals game today.

I think it was the bottom of the 7th, or 8th. There were a couple of close ball calls that went against the Cubs. So after the third out, the Cubs pitcher, argues with the PU. The pitcher and catcher end up getting ejected.

So my question is, when does a replacment for the ejected players have to be announced? With the pitcher being ejected, the Cubs could want different pitchers in next inning, depending if they score in the next inning. I would assume that the sub does not have to be declared untill his spot came up to bat, or he had to take the field.