It could be worse. Consider this - given that for some time, Cohen's phones were being monitored (apparently call logs, not actual speech content), it would seem that there is already a phone communication history record between Avenatti and Cohen in the hands of the Justice Department.
Would that not require Mueller to find out what Avenatti knows or doesn't know? Or is willing to make up, invent or divulge on behalf of his client?
Of course if Avenatti and Cohen did everything via email or in writing instead of via phone, the records (emails, texts, letters, etc.,) are in the hands of Barbara Jones. How much of that she will consider relevant is still in question.