When used properly, Poulsen is a very good player. He always showed that before signing for Juve and he's started to show it for Juve now that he's being given the chance.
It doesn't matter who's underrated or who's overrated. What matters is what we need. And to me, what we don't need is someone who gives the ball away relentlessly. So Poulsen is actually rated higher than Melo and Sissoko right now.
No, regardless of system. Whether you play possession football or sit back for the counter-attack, you can't have someone who gives the ball away or can't make good on the few opportunities to spring an attack that we'd have.