I do get the feeling the 3-5-2 isn't going to work forever.
It is very situational and dependent on the other teams setup and formation. Honestly, I am rather surprised we did not see this from the beginning with him. Both Everton & Southampton under him used 3/5 back systems and he had success with those formations.
It does an excellent job of limiting exposure on offensive transitions and allows the general lack of pace in the squad to be mitigated somewhat. Considering the manner in which we were losing big matches like these (defensive mistakes, being exposed on the counter) it is a sound adjustment from Koeman. Key thing for me too is that it creates more threat for us when we do switch into a 4-3-3. When he subbed Dest for Greizmann, Sevilla had to adjust to that tweak and it was key in tilting the game in our favor.
Against PSG, at the very least, they would not just beat us alone off of a long ball for Mbappe to run on to or a 5 on 3 counter like in the first leg. Their form still is not impressive domestically. We will see, our chances come down to whether or not PSGs midfield can keep control of the match again. If they have long spells on the ball, they can kill the tie off if it’s still 0-0 at say halftime.
On the board from a tactical perspective though, if PSG lineup in a flat 4-5-1 again, the wingers have to go with our fullbacks or we will overwhelm them. It is not just the width in attack, it is the space that Dembeles runs open up for Messi between the lines. It will stretch PSGs MF a lot as well so tactically, Poch does not have a choice but to drop the wingers for the fullbacks and that gives us a 1 v 3 at the back, allowing us a better chance to pin them back. That is our chance here but we need to get at least a goal in the first half. I think it is worth trying in the first half here and then switching into the 4-3-3 like he did against Sevilla to see if how they react.
That said, if we do get that goal and get it early, I can see them unraveling. They did not respond to the goal from Messi well at all and took about 10-15 minutes to stabilize and given the history and our form right now, we should not rule it out right now.
We should not be disappointed if we do not come back against Paris though and I would not get my hopes up too much for it. Even if we do, the CL is not the objective. Being competitive is so I just want us to play well and not get embarrassed again. If we play well and only win 2-0, I will be ok with that.