Because Koeman is clueless. Given the striker situation we should not have been selling Niasse on deadline day.
I'm pretty sure Koeman wasn't happy that Lukaku wasn't replaced and I don't think would have said to the board, 'don't worry, we go with what we have got'.
Think he was let down by not getting the first choice targets, mainly because they didn't want to come. Thinking of the summer transfers of strikers, there was no one who we would want who was good enough to replace Rom, or we simply couldn't attract them.
What RK has to answer to is picking the wrong teams to start with. He's got some good players to choose but he's doesn't have to play them all. That's the idea of a big squad. Competition.
Make your life easier RK and pick a team that is balanced with a strong bench.