When I call him a defensive/supportive player I don't mean he's necessarily a holding midfielder. I consider Matuidi and Khedira defensive players, even if they move all around the field and get into the box sometimes. But their offensive contribution is just supportive, normally they play it simple and remain on a second plane. Their task is mainly physical and protective, more about covering space, to be prepared after a ball loss to put immediate pressure and some other things.
Renato is young and can develop in every sense but by the moment I can't see him as a complete player. Benfica is giving him room to express and see how he responds in every way, and the kid is trying a lot of different things. But I'm not sold with his overall skills, and I think his guarantee now is just his physical attributes and great mentality and work rate. He's not consistent and reliable in any other aspect.
I'd try to sign him anyway if Benfica was reasonable at setting the price, I don't deny he's an interesting prospect. But I wouldn't make a big bet here, when all we need is some energy and muscle to dynamize our apathetic midfield.
We can try to find a future star for midfield, but in that case we should pay attention to more technical players, the ones who control games like Modric or Xavi. Every player can make good isolated actions and get a clip on youtube, but not too many can dictate the tempo and lead their teams as a constant, with intelligence and precision. If we found a young player with that profile I'd even pay money from my pocket. Unfortunately the people at our club go to the beach and don't find the water.