Even if the paperwork is completed in time for the Wearsiders' opening day encounter with Birmingham on August 14, Riveros and Paulo Da Silva have been called up to play in Paraguay's game with Costa Rica just three days before the season starts.
Following their involvement in South Africa, Riveros and Da Silva were given a break from Sunderland's preseason preparations.
But while Da Silva this week returned to Wearside, Riveros has remained in his homeland to train with local side Club Rubio Nu.
The news will have frustrated Bruce who this week claimed the signing of Riveros meant that he was under no pressure to recruit a replacement for Lorik Cana.
Reports of interest in Manchester City's Stephen Ireland were dismissed by the Sunderland manager who had earmarked Riveros to partner Lee Cattermole in midfield.
Bruce may now turn to Jordan Henderson, Kieran Richardson or Jack Colback as it looks increasingly unlikely that Riveros will have a chance to prove his match fitness ahead of the big kick-off.
Similar visa problems delayed Da Silva's arrival at the Stadium of Light by over a month last summer and the defender did not make his Sunderland bow until mid September. He went on to endure a stop-start campaign as he struggled to oust Michael Turner and John Mensah.
But following his assured displays in the World Cup Finals which attracted several potential suitors Da Silva has warned Bruce he would not be satisfied with another season on the sidelines.
After some days of having rest I am going back to England with a big expectation of starting the season, said Da Silva. I am going to join the team in the training sessions and try to get myself a place in the team.
I also know that there have been some proposals, that some offers arrived at the club for me but I think that the club doesn't have in mind letting me go as the league is about to begin. But if an offer arrives and it's something useful for the club and for me, then maybe we'll have some talks.
Da Silva is not expected to be included in the squad that travels to Leicester City tomorrow for Sunderland's penultimate pre-season fixture (ko 3pm). Cash turnstiles are open at the Walkers Stadium for away supporters.