Newcastle have made Sels one of five signings which they have brought in this summer.
The goalkeeper has signed a five-year deal from Belgian side Gent, but it is unclear whether Benitez intends to use him as Newcastle’s first choice yet.
Newcastle still have Tim Krul, Rob Elliot and Karl Darlow ready to compete for a place in Benitez’s first-team.
And the Spanish boss said: “Matz was the best keeper in Belgium in the last season.
“Obviously, we have to wait and see how he adapts to the Championship and the English style.
“He has the physicality. He wants to adapt and to cope with the strikers. You have to be strong. It’s little things you have to adapt. I think he will be fine.”
Sels' arrival could spell the end for Krul, who is widely expected to depart Tyneside this summer.
Elliot and Darlow both showed their quality for Newcastle last season in the Premier League, though - meaning Sels faces tough competition.
Sels made his Newcastle debut last weekend, when he started in goal for Benitez’s side in a 6-0 friendly victory over Bohemians.
Newcastle’s first match of the season comes against Fulham at Craven Cottage next month.