The 23-year-old Spaniard who joined in a deal worth £24million from Villareal last month, won the U21 European Championships last week and is entitled to an extended holiday after the tournament under his contract.
Manuel Pellegrini and his first team squad are training hard in the Swiss Alps with double and triple training sessions. They face their first pre-season friendly on Thursday in Austria.
West Ham will fly off to Asia to take part in the Premier League Asia Trophy where they will face Manchester City.
Fornals has been sightseeing in London with his girlfriend and took a tour of the London Stadium to check out his new home. Arthur Masuaku and Fabian Balbuena are also set to miss the trip to China for the same reasons.