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.
I really imagined the temperatures in Europe were already decreasing.
Even though the world cup is in Qatar, many players may have some difficulty with this high temperature, including players from the African continent or some countries where the temperature is warmer!
By the way, another point that will also affect players (or the most of them) is the thermal sensation and possibly the air being drier in that region!