What's wrong with saying what you typed above verbatim? It is a fairy standard scenario and your wordings probably have been said in one-on-one millions of times. You need to follow up the sentence with "what's next", since the scenario does not have a simple solution (the manager can tone down the demand, but then output and quality goes back to where is was and we have to deal with that). But now that is more about the work itself rather than communication