According to UOL Esporte, Chelsea have refused to let Willian join Brazil's U23 national team at the Olympics, which will be held in Rio de Janeiro in August. Willian is one of the (maximum allowed) three overage players under consideration by manager Dunga. Willian is already set to play in the Copa América Centenário that will be held in the United States next month.
The Brazilian FA haven't given up though as they consider Chelsea's refusal a "preliminary decision" rather than a final one, and thus will try again later in the summer to get their man. UOL Esporte reports that while Willian had told Chelsea's board his wish to represent Brazil at the Olympics, that would mean missing pre-season as well as the start of the new Premier League season, not to mention, a proper rest. Thus Chelsea's reluctance.
Willian's situation isn't unique. The Brazilian FA faced the same problem with Barcelona's Neymar, with the Spanish club refusing to let their player play both the Copa América and the Olympics for the same reasons. Ultimately, after much negotiation, the Brazilian FA decided to have Neymar only at the Olympic Games.
While Chelsea's refusal goes against Willian's wishes, the club are the ones paying his wages and would be the most affected by an overworked player. It's understandable and commendable that Willian wants to play as much as possible for both club and country, in this case, it'd probably behoove everybody involved to choose a more sensible arrangement.