The Ghana international spent last season on loan at Vitesse Arnhem, but will now seemingly spend the 2014-15 campaign in the Premier League.
"Everything has been agreed with Christian. It's just a matter of finalising little details more with his work permit and personal situation," Martinez told reporters.
"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 joined Chelsea from Porto last summer.