You're overestimating the power Mendes has. It's not like De Gea's contract runs out in the summer and he hasn't shown any signs of unhappiness. He won't be able to use him as a bargaining chip.
I'm not so sure of that, he is by all accounts the most powerful man in football these days. Remember how much influence he had at Real when Mourinho was there. Although, I agree that it is a stretch what I suggested there.