Adrian is the number 2. He will let shots in that Alisson could save, he will have a lower pass % than Alisson would have.
Hes about the same quality of Migs I think but it was a no brainer to cash in on Migs and to replace him with someone who would be happy to be the number 2.
I think Mignolet is of the age he wants to play and at the stage of the career Adrian is in this is a great oppertunity for him.
By all accounts he is another big presence in the dressing room as well which is all good for keeping morale and standards high.
With a #2 keeper there is a drop off in quality, with Adrian though like Mignolet (even though he was hated by a lot of the fan base) the drop off is at least not unworkable.