Well yeah something like that, he did in fact have some off days for us playing as a CB. One could argue that his performance at West Ham where we lost 3-2 (last match in their old stadium) is what cost Van Gaal his job in the end, because a draw there instead of losing and we'd have finished one point above City that year. Personally I think Blind made a brilliant choice going back to Ajax, and he's at the perfect place for him right.
What's also fascinating is how little Blind's quite brilliant building up from the back helped us with creating chances in the end under Van Gaal's possesion based team, because we ended with such an incredible low amount of goals in his last season and barely created anything in plenty of matches. Now in this Ajax team Blind's doing pretty much the exact same things, and it really helps to take their team to the next level.
Regarding Mourinho, I think it's quite obvious what he wanted when we didn't sign Davinson Sanchez after the EL final but went for Lindelof instead, he wanted someone who had composure and was comfortable on the ball like Blind only slightly bigger and stronger, and faster of course. At this moment Lindelof hasn't really been an upgrade on Blind though, but I guess he's also 5 years younger.