The victory - Benfica's 10th in succession in the Primeira Liga - moved them to within three points of claiming the title, but their mood was soured slightly by the injury to goalkeeper Oblak.
The Slovenia international clashed heads with Arouca striker Roberto in the 65th minute, after goals from Rodrigo and Nicolas Gaitan had already put the league leaders in control.
The 21-year-old was given lengthy treatment on the pitch before being replaced by Artur, and Benfica confirmed Oblak suffered concussion.
"Benfica can confirm that the goalkeeper, Oblak, suffered a head injury and a momentary loss of consciousness," a club statement read.
Oblak is in doubt for the second leg of Benfica's Taca de Portugal semi-final with Porto on Wednesday, while coach Jorge Jesus will hope to have him back in time for the first leg of their UEFA Europa League semi-final with Juventus on April 24.
