A deal for the 22-year-old Ghana international is still subject to a work permit being received and the completion of other necessary paperwork.
If completed, the move would mean a second successive year on loan for Atsu, who moved to Stamford Bridge from Porto for £3.5million last summer, but then headed out to Vitesse Arnhem.
Martinez told the Liverpool Echo: "Everything has been agreed with Christian - it's just a matter of finalising 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."
Chelsea have not commented on the possibility of a deal.
Meanwhile, Martinez is hopeful one of last year's signings, Arouna Kone, will be fit next month.
The Ivory Coast striker, brought in from Martinez's former club Wigan, has been out of action since suffering a knee injury last October.
Martinez said: "Arouna is training well and is still in his own programme, but he has been working with the team as well.
"We're very positive about Arouna being able to play some sort of competitive football in the next four to five weeks."
Everton begin their Barclays Premier League campaign away at promoted Leicester on Saturday.
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- Arouna Kone