The Ghana international has been linked with the Toffees for much of the summer and is now set to make the same move from which Romelu Lukaku – who has since joined permanently for 35 million euros – profited so greatly last term.
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Martinez has confirmed that the switch is pending only the final details before becoming official.
"Everything has been agreed with Christian," he told the press. "It's just a matter of finalizing little details more with his work permit and personal situation.
"That will probably take a couple of days to get sorted but we have an agreement in principle to make the move as a loan deal until the end of the season."
Atsu, who joined Chelsea from Porto last summer, was immediately loaned out to Vitesse Arnhem, who finished sixth in the Eredivisie in 2013-14 with several Blues starlets among the team. Atsu was voted the club's player of the year by the fans.
Martinez added of possible further reinforcements: "We'll take every day we have left in the window just to assess what we have in the squad and I'm happy with what we have.
"I would say we will take right until the last moment."