~snip~
They will mostly face this issue of high temperature/ hot weather conditions within their few weeks (1week-2weekd) of their exposure/presence to high temperature in Quata, So I do think that adaptation/acclimatization to that environment will start to take place, take for instance people who comes from extremely hot weather environment to play football in Europe ,what happened when they gets to cold weather environment, you will realize that at a particular they got adapted.
Too I think some measures will be in place , maybe FIFA would likely allow up to 5 or more substitution to take place and equally have a brief period where players will have theirselves cooled down a bit.
Most countries in Europe are having similar weather conditions right now, temperatures around 40 degrees. If anything, Qatar will be a bit cooler than Europe right now when the World Cup starts.
I don't think the weather will play a huge role in this World Cup, specially for the European countries.
It will probably be a pleasant and nice weather throughout the entire competition, maybe around 20s and 30s.