The 23-year-old has already racked up 13 appearances in the RaboDirect PRO12 this season with five tries to his name.
And according to Ross, Sexton's move should be seen as an opportunity for Madigan.
He told the Irish Independent: "It is an opportunity for Ian Madigan to step up as he has been doing. He is a different type of player to Jonny; he has his strengths all the same.
"Jonny will get to experience a different culture. Let's face it, Paris is not a bad place to live; he'll probably come back fluent in French and his leaving probably won't affect Ireland, it is more of a blow to Leinster.
"My personal experience of it came at a different stage to Jonny. I didn't have any other option really, but I really, really enjoyed it. I still talk to the friends I made at Harlequins, so it really was the making of me as a player.
"Any young guy who doesn't make it at the provinces, I think they should go. Jonny's at the top of his game, he is at a different level."