When a ball is being played diagonally between the CB and RB, it is 100% the RB's responsibility, there's no ifs, buts or maybes. In that position with three strikers running in between defenders that's just obvious;I disagree. When defending as part of a unit you regularly have to adapt and change markers fluidly in your zone. As said, everyone knows where Mane will run and the centre half needs to ensure he's positioned well enough to prevent it.
Garcia has a similar turn of pace to Otamendi and against Liverpool, and many other elite teams, we need more speed in that position.
Garcia will be a good asset against 90% of other teams.
Mendy was positioned to stop the easy ball down the line, Laporte moving across to get goalside of Salah, Garcia moving across to get goalside of Firminho and Walker moving across to get goalside of Mane. It really is that simple.
Trying to blame Garcia for Mane getting in behind is embarrassing and just shows a complete lack of knowledge or someone purposefully trying to dig out a young player, it's bollocks.