The Chilean suffered a groin injury during Arsenal's clash with Manchester United on Oct. 4, but he managed to play in both of Chile's recent World Cup qualifiers. Even so, Gunners boss Arsene Wenger faced an "anxious" wait to make sure Sanchez was fully fit ahead of the Watford trip, per Sky Sports.
The 26-year-old is one of Wenger's most pivotal players, acting both as a creative force and one of Arsenal's best finishers. He was the team's top scorer in the 2014-15 Premier League season and only trailed Santi Cazorla in assists.
Sanchez has been in blistering form of late, too, notching six goals in his last three Arsenal matches. In addition, he scored three times while away on international duty.
The Gunners aren't lacking for talent, but Sanchez is part of an elite class of players who cannot simply be replaced.