Yann M'Vila is due to link up with parent club Rubin Kazan over the next 48 hours after missing the opening day of pre-season training.
The entire Kazan squad - including those who had spent last season out on loan - were due to assemble today for new boss Javi Gracia to run the rule over them.
But ex-Sunderland loanee M'Vila didn't report for duty, with visa issues holding up his return to Russia.
Reports in Russia have indicated that Kazan are hopeful that M'Vila will be back by Wednesday, yet the delay will raise further question marks over the Frenchman's future.
Both Sunderland and M'Vila have made no secret that they want to reunite next season after the midfielder's hugely successful season-long loan spell at the Stadium of Light.
And Kazan accept that M'Vila has no future at the club, with only six months remaining on his contract.
But M'Vila will either have to reach a settlement with Kazan to tear up his deal, or Sunderland will need to make an acceptable offer for a permanent contract.
Sunderland would also have to agree personal terms with the 25-year-old, who is on a huge deal at Kazan, thought to be in excess of £100,000 a week.