Concussions, as we've talked about here before, are an extremely tricky thing. Some concussions a person can bounce right back from, others linger and take forever to shake off. In the case of Dries Mertens' concussion, which appears to have been more serious than we were first lead to believe, getting all the way "right" after this one is going to take some time.
According to reports in Italy, Mertens is still suffering the after-effects of his hit. He's been experiencing persistent ringing in his ears and is still have balance issues while trying to walk. That all adds up to "not nearly ready to play," meaning it will take a not-so-minor miracle for Mertens to be fit to face Cagliari on Sunday.
His absence will likely be longer than just that, though. Considering the symptoms he's suffering and how they'll hold him back from even being able to train for the time being, it'd be surprising if he's deemed ready to play Sparta Prague next Thursday either, especially considering that match will involve a trip to the Czech Republic. In fact, don't be surprised if we don't see Mertens well in to December, in order to give him as much a chance to get healthy as possible.
If Napoli want to make sure Mertens doesn't suffer further ill effects or a more serious injury from another blow to the head, they need to give him the time he needs to get right. Losing him hurts, especially with Lorenzo Insigne out, but it'd be better to have him miss a couple of weeks now than months later down the line, or worse, have his career end because of another concussion. Rafa Benitez is a smart manager, he'll figure out how to keep the side ticking along without him in the meantime.