Meanwhile, the Gunners will be looking to be one of the main contenders for a top-four finish next season as they bid to try and break back into the Champions League spots for next term.
Sanchez has won 88 caps for Colombia and played in the Premier League for Aston Villa between 2014 and 2016.
The defensive-minded midfielder becomes the ninth player through the door at West Ham during a productive summer window for Manuel Pellegrini.
Oscars: Academy to add achievement in popular film category
Essentially, with these changes, the Oscars are looking to become more like the MTV Movie Awards, and that's a damn shame. It's a way for the Academy to include movies they probably wouldn't recognize in the first place (see: genre pictures).
His lack of game time under former manager Arsene Wenger saw him make the loan move back to Deportivo, where he scored eight league goals in 35 matches.
"I'm a dreamer", Hernandez told the club website ahead of Sunday's trip to Liverpool. He spent the second half of last season on loan with Espanyol in La Liga.
The former Rayo Vallecano forward paid tribute to the Arsenal supporters in a message on his Twitter account, saying: "They always showed me affection and love from the first time I arrived in London".
'I'd like to thank all of the people, the whole club, for having me'. West Ham director of football Mario Husillos moved for Sanchez following a ringing endorsement from the Colombian manager Jose Pekerman under whom he has reached nearly 100 global caps.
West Ham director of football Mario Husillos, who added Perez's name to a list of signings that includes Yarmolenko, Ryan Fredericks, Lukasz Fabianski, Issa Diop, Jack Wilshere, Fabian Balbuena and Felipe Anderson, underlined the Spaniard's versatility.