We weren't always a good counter-attacking unit under Poch. In fact, in the classical sense of the term, where counter-attacking involves ceding possession to your opponent but hitting them on the break, I'd suggest that we've really only come into our own as a counter-attacking unit *this season*.
14/15 - we were a mish-mash of approaches, only unified by fierce desire and a determination to compete (as evidenced by the number of late wins and draws we salvaged). Otherwise, we were very much a work in progress.
15/16 - we had basically one way of playing that season, which was aggressive passing in forward areas twinned with a relentless hunger to win the ball back when we lost it. We were nearly always the team with more possession - but when Plan A failed, we were pretty toothless.
16/17 - we merged the 15/16 style of play with a bit more ruthlessness and more street smarts - and although we made our first attempts at being a pure counter-attacking unit that season (switching to 3-at-the-back against Arsenal at the Emirates and looking to hit them on the break, as one example), we were still more comfortable with dominating (or at least shading) possession than we were with ceding it. And we still stuck to our ideological guns a bit - remember, Poch played Klopp's Liverpool twice and set us out to be the superior side in possession, but we were outplayed both times (and lost once, at Anfield).
17/18 - this is the first season where we look more comfortable out of possession than we do *in* possession - against Dortmund, against Liverpool and against Madrid (twice), we have ceded control and possession in exchange for sheer ruthlessness and speed on the break. This is encapsulated by the difference between Liverpool coming to WHL last season and them coming to Wembley this - last season, we were dominated in the first half, and then endured a pretty turgid second half only livened up by Rose thumping in a rare chance for the equalizer. This season, we played Liverpool like a smaller side would, and utterly obliterated them.
So, overall, I'd say we weren't a counter-attacking side under Poch until very, very recently. And Sissoko does help with that. One of the first things that comes to mind when I think of his performances this season is the speed with which he breaks when we're in possession (or he's on the ball) - visceral speed allied with pace. Even last season, we had basically three players that could do that - Son, Walker and Rose. This season, Walker's gone, Son hasn't played much and Rose has been out injured for most of it. Thus, Sissoko has at times been our only really pacy option - which is pretty damned central to a counter-attacking side, wouldn't you say?