I read a lot of posts by people wondering / questioning Klopp's substitutions at half time for our game at the Etihad. For me it did make sense to take of Salah and I'll try to give my point of view with regards to why Klopp took of Salah and why he changed our formation. This is all just my own thinking, it might be completely wrong. Feel free to disagree and add your own views and ideas.
All I'm about to say really only applies from the start of the second half to the moment when we conceded the 3rd goal and he took off Firmino. Everything that happened after that, isn't really worth discussing. For me, that was the moment Klopp 'gave up', not during the break when we were down 2-0. This is about the idea behind the changes, not really about the execution.
The main debate was why we didn't use two banks of four and try to sit deep rather than go to three at the back with wingbacks and still try to maintain a relatively 'high' line.
I know it looks a little amateurish, but it's the best I can do and it helps visualize things.
If you look at the players we had left on the pitch and the bench, we just weren't able to play two banks of four. For that to be effective, you need different players (Preferably two 6's and two quick wingers). The way we defend and set up, is to control space by pushing up high on the pitch and keeping the pitch as small as possible, control the passing lines and counterpress. To go from one extreme, front foot defending, to another, sitting deep in two banks of four, is a big difference that can't be adjusted in a break of 15 minutes. We train on fast transitions day in day out. From defence to attack and the other way around in a matter of seconds. We don't train on 'parking the bus'. Teams that specialize in this train on it every week, with eleven players and a completely different set of players. Even these teams tend to struggle with the two banks of four set up against City, certainly at the Etihad.
There are three main talking points:1. The Salah -> Chamberlain substitution
While Salah was indeed the one who offered the biggest threat on the pitch by trying to get him in 1 vs 1 situations with Otamendi, that threat would have easily been nullified had we reverted to a 4-4-1. Salah isn't good enough defensively to play this deep as a 'traditional' right winger and he would have been too close to City's wingbacks. He would have to start his runs from too deep and be up against it with very little support other than Firmino. We didn't have the runners in midfield to offer support without leaving the midfield wide open (Henderson or Can in this case). 2. City creating overloads in wide areas
This is actually what we struggled with the most. City vacated the centre of midfield. Only Fernandinho stayed central to shield the three CB's. Both De Bruyne and Silva were often going into wide areas to support their wingbacks and create overloads on the wings. Trent was always up against both Mendy and De Bruyne which created a lot of problems for him. This of course is also down to a lack of support from our midfield in these areas, but they were also occupied by Aguero and Jesus taking in the pockets of space we left in midfield. Going to a 4-4-1 wouldn't have solved this problem, it would only have made it worse because the spaces would be even bigger in midfield.3. Trying to score a goal on a counter
This I think would have been nearly impossible in a 4-4-1. Both fullbacks would be too deep to offer support, Salah I explained in point 1, no support from midfield without leaving it wide open, a lack of passing options for whoever was running up the pitch with the ball and Firmino lacks pace to do it on his own.
This is how we set up at the start of the second half. This formation would allow us to control the spaces better and still try and play something similar to what we are used to playing. Klopp doesn't like his team in straight lines, which two banks of four usually is, he likes asymmetry in midfield.
Same talking points:1. The Salah -> Chamberlain substitution
Alex offers a lot more defensive stability than Salah, while still offering a lot of pace and the ability to carry the ball up the field. With him we would have been able to keep the centre of the field closed and still force City to go into wide areas, which they were doing in the first half. He would also be able to carry the ball up the field from a more central position, offering him more passing options rather than Salah having to start from deep on the wing. 2. City creating overloads in wide areas
By going to three at the back, both our RCB and LCB would be able to get over to our fullbacks (wingbacks in this formation) quicker to offer support with the overloads, while also having both Wijnaldum and Alex in midfield being able to provide help on the wings. This last thing would be a lot more difficult for our midfielders in a midfield two. 3. Trying to score a goal on a counter
The starting position of our fullbacks is now higher. City were still pushing high up the pitch with both their wingbacks deep in our own half. We would be able to attack the same space in behind their wingbacks and create 1 vs 1 situations (which was the plan we started with). Both Moreno and Trent would be able to beat Danilo and Otamendi with their pace. Whenever we would get the ball, you could see either Moreno or Trent pushing high up the field immediately (We actually created a chance like that with Trent). With this formation, we also had Alex offering support from midfield, without leaving the midfield wide open and still keeping at least 5 players behind the ball (the 3 CB's and the 2 remaining CM's).
Now all of this is of course 'in theory' as clearly the plan didn't work out and Klopp started changing it after we conceded the 3rd, which in my opinion is down to Can not being used to playing in the middle of the back three (he did play there as a LCB or RCB) and completely messing up his positioning so he couldn't intercept the ball.