Casemiro and Martinez have been pictured at Carrington in recent weeks, sparking hopes that they might make it back into the matchday squad for some part of the festive period.
But the manager was more cautious in his pre-match press conference ahead of the West Ham United clash (Saturday, 12:30 GMT).
Asked to expand on the likely return date for the South Americans suggested in previous media briefings, Erik said: "They are not available before Christmas. Mid-January we expect them back."
Another question followed on midfielder Mason Mount, who has not played any minutes since the 1-0 win over Luton Town on 11 November.