Trent Alexander-Arnold is a very talented center mid who's been slotted in at RB more or less out of necessity. He's done extremely well there but he's 19 years old, so he's more or less been asked to stay at home and be a defend-first fullback, at least against good attacking teams. His delivery is a bit inconsistent but when he's on, he's got one of the better crosses on our team.
Given the high stakes of the match and the quality of the opposition, I wouldn't expect him to get forward a ton, unless that's specifically something Klopp will try to exploit. I would think that with Salah ahead of him on the right flank, that will be enough of a threat to give Marcelo nightmares and that TAA will fill in the space but not do much in the overlap.
Andrew Robertson, on the other side, is a very, very good attacking left back, however. Generally if one of our fullbacks gets in the attacking third, it's him. He's the Scottish Roberto Carlos.(Kidding. Sort of.)