The 26-year-old, who has more than 60 caps, was included on Tuesday in Bento's 23-man squad for next week's friendly against Netherlands on the Algarve but the coach said he was no longer a first-choice starter.
"He will now have to put in a bit of effort to recover what was once his - the role of playing nearly every time," Bento told a news conference.
"That did not happen in the last few matches. He needs to do his bit and then we will see what we will do."
Cape Verde-born Nani gave way to VfL Wolfsburg winger Vieirinha in the last few internationals, including a precious 1-0 win over Russia in June that sent Portugal to the top of the World Cup Group F qualifying table.
"He (Nani) has had to cope with Vieirinha who did extremely well in our recent matches against Azerbaijan, Israel, Russia and Croatia," Bento said.
Nani missed United's pre-season tour of Asia after having an operation on his nose.
"We still called him in because of his great quality and potential," Bento said.