The Slovenian goalkeeper's injury took the gloss off Benfica's 10th successive league win, which left them three points away from wrapping up the title race.
Oblak clashed with Arouca striker Roberto in the 65th minute and received lengthy treatment on the pitch before being led away to the dressing room looking groggy.
Benfica said in a statement that he had suffered a "momentary loss of conscience."
The 21-year-old, who began the season in the reserves, has been given a run in the first team after Artur suffered a loss of form at the start of the year.
He joined Benfica in 2010 and has since been loaned out to Beira Mar, Olhanense, Uniao Leiria and Rio Ave.
Benfica went ahead just before halftime when the Arouca defence failed to cut out Lima's cross from the right and Rodrigo touched the ball home from close range.
Lazar Markovic left several opponents in his wake in a run through midfield before laying the ball off for Nicolas Gaitan to chip the ball in from the edge of the penalty area for the second in the 54th minute.
Benfica have 70 points from 27 games and will clinch their first title since 2010 if they win at home to Olhanense next Sunday, or the day before if second-placed Sporting lose at Belenenses.
Sporting have 63 points after beating Gil Vicente 2-0 on Saturday.
Carlos Eduardo and Juan Quintero scored in the last five minutes to give third-placed Porto a 3-1 win at Belenenses on Sunday.
- Sports & Recreation