True that winter in qatar is still not as people would expect but i don't think the temperature will be that high plus most of the players are professionals i don't think this will bother them much i think the high quality of the stadiums and infrastructure in QATAR will make them forget about it, i still think this will be the best world cup in terms of organization.
Maybe it will still affect how they play but most of the athlete that I know since they are competing outside their country, they will go to that country a month or 2 so that they can adjust to the weather and their environment especially if the country they compete is very different to their home. If I am not mistaken conditioning is their term every time they flew away so they can adjust.
It seems that it will have an effect because they are not used to the weather they often feel, and of course there will be adaptations for players so that they can still play well and also maximally.
considering it's a prestigious world cup, I think it's all set up to adapt to the cold weather so it doesn't affect it, and it's possible that the team shirt is made of cold-resistant material and has long sleeves. And maybe wear a mentset in addition.
I think the clothing department will take every step necessary to make sure that the players play at their 100% in every match.
But, the change of location and weather is always going to affect any athlete at any time. often we forget about the Jet lag which at times can prove to be a really bad thing for any player. But that obviously does not affect the players directly as the weather does. I don't think it will be that much of a problem because most of the players are used to playing in different conditions.