The midfielder has endured mixed fortunes this season but put in a stellar performance at the weekend as he scored his first Premier League goal of the season in the win over Spurs.
And Lage says the Belgian is an essential part of his squad and sets an example to the rest of his players.
"Leander is an important player for me," Lage said.
"Every time he is there, when he starts, when we play with three midfielders, when he comes from the bench and helps the team.
"That's why I'm so happy, because he's working hard every day.
"He's a good example of what a team-mate should be. Even time he is available to work, even as a centre-back.
"He is a top man, he is a top player and I am happy because he deserves this goal."
Dendoncker receives criticism from some sections of the fan base but Lage says the 26-year-old handles that pressure well.
"He's a big character, since the first day," Lage added.
"I never talk about this with my players, but when you have a top player like him and I'm Portuguese and Ruben and Joao are also Portuguese, the first thing you could put in your mind is that they will (play ahead of him), but no.
"He is there, understands the role I have for him. He can play in the first, second and third position.
"He is a top player and a top man with the right mentality."
Dendoncker's goal came after a good interception from the midfielder before he anticipated the danger in the Spurs box to slide in and finish.
His team-mates, too, appreciate what he offers on the pitch.
"Leander's performances always go under the radar," Max Kilman said.
"He does the job and gives you a good performance every time. I'm delighted for him to get his goal and glad that people realise how good he has played.
"He's brilliant, nothing phases him. He knows his task and he doesn't feel the pressure. You can see that when he performs on the pitch.
"He is very solid and helps us a lot."