The inadvertent admission by Perez at Interlagos came following Sauber's announcement that it would reveal its 2013 driver line-up by the end of the Brazilian Grand Prix weekend.
It has already signed current Force India driver Nico Hulkenberg for one of its seats.
Incumbent Kamui Kobayashi's position has been in doubt for several months as he hunted for more funding. At Interlagos on Thursday the Japanese driver conceded he might not be on the 2013 grid.
Gutierrez and Perez share the same Mexican backers, including Telmex, as part of the country's efforts to establish itself as an F1 force.
Asked by Spanish-speaking journalists how competitive he expected Sauber to be in 2013, Perez cited a Hulkenberg/Gutierrez driver line-up as one of its key assets.
"The car will be strong because the rules don't change much, and the team have a good experienced driver in Nico and he will have a strong team-mate in Esteban," said Perez.
Pushed to confirm that meant Gutierrez's Sauber race deal was finalised, Perez laughed and said "we are waiting for the announcement."
Gutierrez made his grand prix weekend debut in Friday practice in India when Perez was unwell.
A former GP3 and Formula BMW Europe champion, he has been in the Sauber fold for several years, having first tested for the team in 2009 as his reward for his FBMW success.
His past two seasons in GP2 with the ART/Lotus team did not live up to expectations, but the 21-year-old took three wins on his way to third in this year's standings.