For the players it should not be a concern. Because most of them are are participating in club championships around the world, such as the English Premier League. In fact some of the top club sides wanted the FIFA to postpone the tournament, so that their championship leagues could happen without much disruption. So the fitness levels should not be a concern, although adjusting to the weather conditions in Qatar can be one of the topics for scrutiny. For experienced players, this should not be a big worry as players such as Messi and Ronaldo have played around the world in different climatic conditions.