The knock is a blow for the Cameroon international, as he started the club's last two matches including scoring the winner against Everton.
Job twisted his knee during practice and will now see a specialist in order to determine the extent of the damage.
The 25-year-old will definitely miss this weekend's clash against big-spending Chelsea, but could return to face Newcastle.
On the plus side for Boro, Italian hit-man Massimo Maccarone is set to make his comeback from injury.
The former Empoli ace had been selected in the club's reserve squad along with Michael Ricketts to face Sunderland.
