That's a fair point mate. Most managers do have a preferred system. I think Koeman is unique as what differentiates him is his tactical flexibility.
I also suspect with our defensive frailties we will use 3 at the back as quite a stock option this season. I don't think Baines or Coleman are defensively solid enough for Koeman. I don't think he trusts our wingers to player wife players in front of a back 4. I also suspect he feels Lukaku isn't fit enough to play up front alone.
This can be solved longer term with major surgery but helped shorter term by a formation change.
This doesn't mean Jagielka stays. I thought he was good yesterday. However so was Mori. I think after last season I would not object to any player being sold.