It's nowhere big of a risk. Saul is still 26 and in his prime physically, Pjanic is washed up and was never athletic. Saul just likely experienced loss of confidence and form, which he himself spoke about.
Pjanic started declining in his prime...
So did Eriksen....
"Decline" isn't an exact science in sports. Point is, Saul hasn't been himself for about at least 2 years now.