Ozil was a completely known quantity by anyone that watched him for Germany or Real, I said before we bought him, when we bought him, and now, he disappears, he has a crap attitude and never fights hard. Mourinho didn't trust him, Ancelotti/real in general didn't trust him. 42million doesn't magically make someone good, a transfer price is about the people who make the deal, not the player. He had the "Arsenal" mentality to begin with, exactly the wrong type of player and Wenger as per usual made the wrong decision.
He's not strong enough to be good running into the box as a withdrawn striker/ultra offensive central mid, he's too lazy and weak to be a proper central midfielder, he's not fast enough nor strong enough to be a winger.
Di Maria last night cut through barcelona, Ozil in the same situation would have gone down of his own accord at the first man in each of those runs. He doesn't have to be the same type of player as Di Maria, but it just highlights where he can/can't play. he's no winger, not suited to central midfield against good defences of midfields, which is why he's overrun, out muscled, and disappears against good teams.
Sagna is a completely terrible defender, as is Gibbs. Mert/Kos are decent enough, Kos panics a hell of a lot and certain teams have exposed Mert's weaknesses, the only shock is more teams don't notice it and play the same way against him.
Gibbs was the worst player on the pitch by a massive margin against Chelsea, he was at fault for the first two goals and again out of position for the situation leading to the penalty.
I think a new manager can effect mentality, Martinez this year has done so, but old lingering problems still remain. Everton have looked like a different team for the majority of this season... yet when it came to Liverpool the timid "we're going to lose" last 5 years Everton team showed up. Considering they are Everton's biggest rival and the losses would have been the hardest to take it's not surprising that it's the situation that will be the hardest to "fix". Yes Liverpool were great but Everton was woeful. Also don't forget that Martinez got a new striker, new wingers/strikers and replaced the central midfield pairing completely. I'm not so sure their new attitude would have been so evident on the pitch using almost entirely the previous squad. He's changed 4-5 of the starting 11 to achieve this.
While I think a new manager can help the mentality issues, some fundamental changes will be required also.
Ozil has no place at any top four club, lazy, weak, shockingly poor attitude. I've literally never seen a player so unwilling to work the ball. If he gets caught on the wing without much support he'll get to the line then flop over praying for a freekick, other players do this now and then, but it's Ozil's default play, he does it so often it's embarrassing.
We've conceded quite a few goals directly from him simply giving up, flopping down and asking for an undeserved freekick where the opposition just runs at our goal.
Giroud is awful, nothing short of awful. Any striker could score the goals he does, in the games he does, it's the work of the rest of the team putting it into the 6 yard box, 99% of strikers would have a similar strike rate with the chances the team creates for him. He creates exceptionally little for others, and his general play is abysmal. Shouldn't be at the club, if we buy a top notch striker, he does not make for a competent back up striker. Arteta has also completely lost his way at Arsenal and is frankly not going to improve significantly at this point.
We need, 2 minimum, potentially 4 fullbacks, 2 world class 2 decent back up. We need two DM's, one top notch, one back up. One issue with Flamini is while the right attitude and not at all bad, he's been a sicknote for several years and frequently unfit this year. We need a world class DM and a back up, giving us the option for new great DM + flamini in tough games, and having a back up for either being injured. We need 1-2 wide players. Caz, Ozil, Ramsey are crap and ineffectual out wide, and we need 2 to 3 strikers. 2 top notch and a back up.