The problem is that the teams don't have a lot of preparation time for this tournament. Otherwise you could fly to Qatar 3 weeks in advance and then you could prepare optimally there to also adapt to the weather conditions. For example, if you play football in England, France or Italy in November, it can already snow or freeze. Quite strange when you are in Qatar 2 days later on a training field and that it is 32 degrees there. Your body has to get used to that too. What many people don't realize is that you can get a reverse version of a kind of flu due to the extreme temperature difference. If players go home, you can easily catch a cold again in December.
At the moment with the heat wave in Europe you're getting temperatures hovering 40 degrees. That's actually hotter than what's going to be in Qatar for the World Cup.
I think most players will be fine regarding the weather. It's not going to be as hot as summer time there.
It will still be very hot in Qatar, especially as it will almost be winter in Europe by this time. Therefore, it will be harder for European players to adapt than those from Africa or South America. By the way, this is one of the reasons why I think Brazil or Argentina could win the World Cup.