Lacking in talent? You can't be serious?
The squad he took over included players who have since been sold to City, United and Barcelona, and another who would have gone to Chelsea if he hadn't decided not to. He then spent about £100m on new players, so if we were seriously lacking talent he has to take a lot of the blame.
People are criticising the use of 2 defensive midfielders because it led to us lacking control in the middle of the pitch. We couldn't use the ball well enough or move it quick enough, many people feel that sacrificing one of the DMs would have allowed us to exert control in the middle of the pitch, and thus relieve much of the pressure on the centre halves. It may also have meant we could score more than 2 ourselves, and mean the 2 we conceded were irrelevant. Football isn't as black and white as more defensive players = better defence, a balanced side which plays to its strengths is key.
Finally, what could he do about the centre halves? Not buy Williams perhaps? Spend some money on a new centre half instead of blowing 90% of the budget on 3 players for the same position?
Yes, in the second half of the year, that Everton team lacked talent due to various reasons. Joel Robles in goal. Gana and Morgan hurt quite a bit. Williams playing, although he wasn't this bad. Holgate playing. Mirallas. Valencia. It was the Rom and Barkley show. Davies was there. Finishing as well as he did with that lot was IMO impressive.
Now I disagree with your assertion about the 2 DMs. The 2 DMs didn't cause a lack of control in the middle of the pitch. We had complete control. Dominated possession. There wasn't ANY pressure on Williams and Holgate that I saw. At least none that wasn't self-created. Those DMs broke up plenty of counter attacks that would have exposed the center back, so they alleviated pressure on Williams and Holgate, not the opposite. Frankly, playing 2 DMs DOES play to our strengths, does it not? Beyond that, it's not Gana or Morgan's fault that there isn't any movement in front of them to pass to. That's the issue more than playing 2 DMs IMO.
I do admit and understand putting more pressure on the opposition and scoring more goals. I'm not against the idea of switching one out for an attacking player. Koeman did make that adjustment and it worked somewhat, even if it didn't result in more than a single goal. But even with 2 DMs, these guys haven't played much together and it's obvious we are lacking a CF and some fluidity - as a result, I can forgive only scoring 2 goals (barely, though). It's hard to score goals, it's going to take a bit of time for us to figure out the right combination of folks, especially against a team putting 10 people behind the ball. But to concede 2 goals against that lot? Come on, you gotta admit that's a much more shocking and worse crime than only scoring 2.
Obviously this team having another CB and CF, like Koeman wanted, would be a bit different. Williams' obvious regression was one that I think Ronnie predicted, but that's on Walsh and the board, not Koeman.
Regardless, you can't hold Koeman accountable for not buying another CB. There was also some circumstance - injuries to Mori and McCarthy as well as Ross being stubborn hurt our budget as much as anything. That's probably 60-70M in sales that we could've used to easily get a CF AND a CB. Might have been able to spend quite a bit more beyond that depending on who you believe. But that's still the fault of Walsh and the board moreso than Koeman.
So my point is that Koeman is responsible for tactics. He deserves to be ripped for some of it. But he can't conjure up new CBs out of nowhere.