Any solution to the midfield problem has to involve Xhaka, that is for sure. And if he isn't part of the solution, it certainly means the buy was a bust.Xhaka is really the natural Cazorla replacement (both during Cazorla's injury and long-term). Central midfielders with his range of passing, athleticism, and tackling ability are rare and there's a reason he cost 35m. If Cazorla going down submarines their season it's because Xhaka turns out to be a bust, and it will be Xhaka's failure, rather than Cazorla's injury, that's the real problem. Arsenal can afford to buy 35m players but they can't afford to waste 35m on players who don't contribute.
The problem is that he isn't really a like-for-like replacement for Cazorla so there are some significant questions about whether the team has an appropriate CM partner for him. Cazorla roamed around the field helping make things tick with his passing and with his dribbling and excellence in tight spaces. He also often comfortably quarterbacked the attack from 25-30 yards out when we had opponents deeply pinned, moving the ball around and looking to start quick combinations. He is a poor man's Xavi/Iniesta hybrid, and he worked best when paired with an energetic pure destroyer like Coquelin who could mop up behind him. Xhaka is more like a poor man's Xabi Alonso. He is comfortable playing much deeper and spraying passes around and through the lines, but he isn't going to do much dribbling or get us through tight spaces and he isn't the midfielder you want as the main offensive threat in a CM pairing unless your team is going to set up very defensively. Ideally, I'd say you want to pair him with a very active box-to-box player who can provide some real offensive threat making late runs but also has the engine and tactical nous to contribute defensively and not get caught out too often (I don't think Xhaka is mobile enough to be a lone DM with his partner always getting caught up the field). To continue the Xabi Alonso analogy, more Sami Khedira at Real than Javier Mascherano at Liverpool. On paper, Ramsey's skill set should fit the bill. On the field, he really hasn't been very good for about two years now and has shown a penchant for Hollywooding around and being a very undisciplined tactical player when in central midfield.
In sum, there is another very real possibility which is that Cazorla going down submarines the season but its not because Xhaka is a bust so much as the lack of a good partner for him.