In the unlikely event that both teams lose by identical scores then 'fair play' regulations, the extra tie-breaker that FIFA have brought in for Russia 2018, would also be needed to decide which nation progresses to the last-16.
The Iberian pair both have four points from their first two matches, one point ahead of Portugal's upcoming opponents Iran.
Morocco, who face Spain at the same time on Monday, are already eliminated, having lost both their matches so far.
Following the 3-3 draw to open their World Cup campaigns, Spain and Portugal both secured 1-0 wins over Iran and Morocco respectively to go into the third group game with the same number of points, goal difference and goals scored.
As the match between them ended in a draw, all tie-breakers related to that game are irrelevant.
If this remains the case after the final match, then the sides' fair play records will be used.
Spain currently sit atop Group B because they have a better disciplinary record, with only one yellow card from their first two matches.
Portugal, thanks to a 92nd minute yellow card given to Adrian Silva against Iran, have had two cautions.
Therefore, as well as looking to secure the win that would secure safe passage to the last-16, both Spain and Portugal must also look to keep their cool and not add to their card tally.
Under FIFA's fair play system, a yellow card counts as minus one point, while an indirect red is minus three. A direct red card would cost either team four points.
In the extraordinary event that both teams also finish with identical disciplinary records, then FIFA will draw lots to decide the team's placing and, potentially, their World Cup future.
In 1990, the Netherlands and Ireland were tied for second place in their group with identical records so a draw was made to decide who would progress in second and who would go through as a top third-place qualifier.
The Dutch were drawn in second place and the Irish were given a third place ranking.
