The Belgian teenager is expected to complete a move to the Premier League in the coming days, although the Toffees boss sees the 18-year-old as one for the future rather than the first team.
He told the club's official website: "David is only a young man and we believe this is the perfect environment for him to develop and fulfil his potential and we would love to find a way to sign him but that doesn't mean he's going to go into the first team at all.
"He's a young promising footballer and we think he would be perfect for the group of Under-21s that we have created."
Henen is highly rated in Belgium and spent the last year on loan in France with Monaco B, having also featured for his country at junior level.
Meanwhile, after watching his side continue their pre-season campaign with a 1-1 draw against Porto on Sunday, Martinez also confirmed his intention to bring a winger to Goodison Park before the Premier League season begins.
"Hopefully we will have another new player coming in," he added. "We are looking for a wide player that is going to help us in a position that we need to strengthen going into the season.
"We are going to take as long as we need but hopefully it will be earlier rather than later."