The Mexico global scored just eight goals in 28 league appearances last season but is looking forward to working with Pellegrini.
Pellegrini has overhauled West Ham's squad this summer breaking the club's transfer record to sign Felipe Anderson from Lazio while signing Jack Wilshere, Fabian Balbuena, Andriy Yarmolenko, Lukasz Fabianski, Ryan Fredericks, Issa Diop and youngster Xande Silva.
Sanchez, who has over 80 caps for his country, spent previous year on loan at Espanyol.
Perez joined Arsenal in August 2016 from Deportivo la Coruna but started just two Premier League games for the London club, making another nine appearances off the bench. He went back to Deportivo on loan last season and was surplus to requirements at the Emirates after the arrivals of Alexandre Lacazette and Pierre-Emerick Aubameyang. "I am always aiming as high as I can and if we can put West Ham in the Champions League why not?" "I'm very motivated." Perez said.
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Arsenal is reducing on the number of players in its record books to cut on their wage budget which is now high since the gunners do have players like Mesut Ozil who are getting paid huge sums of money so Lucas Perez joins that list of players who get axed from the club.
"I'm very excited, as West Ham are giving me the chance to stay in the Premier League and I'm very happy to be part of this new project".
"We welcome Lucas to West Ham United".
"He is a very experienced player that Manuel and I both know well from Spain".