I heard Juve is going for De Gea and a couple other United players this January. Could this be an opportunity for Ronaldo (and his agent) to engineer 'a return'? 
Actually, the rumor bounced here in Italy too!
The good part of this rumor is that if he will come here it will be a free player and he will cost 0.
His contract will end next year!
So Juventus can sell Szczesny for 35-40 M https://www.transfermarkt.it/wojciech-szczesny/profil/spieler/44058 and take DeGea for 0.
What's about of salary?
Szczesny as today cost at Juventus around 8m with taxes.
DeGea now gains 11.6 m without taxes from ManU.
If he keeps the same salary for Juventus will be 11.6+25%(11.6)= 14.5 m with taxes.
So basically DeGea will cost 6.5m /y more then Szczesny, but Juventus can take around 40m from the sell.
Now if we do 40/6.5 we will have around 6 years if DeGea comes and signs a contract of max 5 years this operation it will +EV for Juventus.
After all those math done, i still believe that De Gea would be a great addition to Juve.
Sczencny is a valuable player and has saved Juventus in many difficult matches but the difference in those prices are not so of a deal for Juventus.
The price difference for Juventus is very important. If you get De Gea for free and sell Szczesny, of course that is an absolute advantage for juventus.
But, what is said by @Naida_BR has a point. That it is also possible for Juventus to get de gea and Szczesny to stay together, it will be even more beautiful.