The Estadio Nacional Julio Martinez Pradanos hosts could not break down the visiting San Marcos in the opening 45 minutes, but coach Martin Lasarte deployed the game-changing substitution at the break.
With Gonzalo Espinoza sidelined with a broken collarbone, Universidad needed some midfield grunt - and Gustavo Lorenzetti's inclusion for the second half gave the home side an edge.
Nine minutes later, Universidad hit the front when Rubio converted from a cross in from the left, before San Marcos' fate was virtually sealed when goalkeeper Pedro Carrizo was sent off in the 74th minute.
Rubio's second goal, in injury time, made sure of the points for Lasarte's men, ahead of their El Clasico with Colo Colo on October 19.
O'Higgins (15 points) were the highest-ranked side to stumble at the weekend, and it cost them three spots in the table as they fell to eighth after a 1-0 loss at home to Palestino.
