The 26-year-old's initial application for a work permit is certain to be rejected, and the Teessiders are hoping for an appeal hearing on Wednesday, provided they can tie up the deal.
Southgate said: "There are still a few bits and pieces to be done on the deal, but I understand that it is not too far away.
"Hopefully we will get him across for a medical over the next 48 hours.
"Obviously we are aware of how many games he has played for Brazil and we expect to have to appeal. We believe that we have a very strong case and we are hopeful he will receive the work permit."