Ibrox boss Walter Smith confirmed Mendes would undergo a medical with the Portuguese giants, who have been chasing the 30-year-old all month. Mendes joined Rangers from Portsmouth in August 2008 on a three-year deal and helped them win the Scottish Premier League and Scottish Cup double last season.
"From Pedro's point of view, he wants to make a push for the World Cup and it's an area where we have good cover. While it doesn't suit us to lose another player, we appreciate his circumstances. It suits both parties."