For the last several years, Witsel actually has been that number six type for Belgium. He sits just in front of the centre backs, doesn't venture forward himself and doesn't put a lot of risk in any of his passes. He's been criticized for slowing the game down too much (walking with the ball for a bit when he receives it and probably passing it to a full back rather than forward). He says that he still plays as a box-to-box for Zenit, but apart from a couple of European matches against lowly Belgian competition, I haven't seen him play for them so couldn't say how he's doing for them.
For Belgium at least, he's basically James McCarthy. He's more athletic etc. but he plays the game the same way. He should be partnered with someone who has more verticality to his game, who passes forward more directly, etc. Besic probably fits that part more than McCarthy and Barry, but is he actually good enough? Maybe Tom Davies could be that player, I'd like to see him in midfield rather than at rightback in the rest of preseason.