In the first place, you shouldn't be buying players just for the sake of buying and then not knowing who to play. Buy who you need.

Why did you get him in the first place if he was not going to replace your current GK?

IF the reason you got this GK was because your current GK was not performing then, you should play your younger keeper.
IF the reason is because you want to power train him with an eye for the future, then play your higher quality keeper.

You can rotate for cup/cl/league also if you wish.