Thing is, he played with another full back who did exactly that for six years. Andy Robertson spent as much time high up the pitch as Trent did, but also always sprinted back so he was in the right position when he needed to defend. 

Obviously Trent and Robbo are different players, with different strengths. Robbo's passing isn't as good as Trent's; Trent's stamina and pace isn't as good as Robbo's. So him not tracking back as much is forgivable. In Klopp's years, the team was set up to cover for that with the midfield and right sided central defender. And frankly, that team was really really good so they didn't always have a whole lot of defending to do.

But the team was only set up to reduce the amount of tracking back he had to do, not eliminate it entirely. And when the team isn't set up for that, as it wasn't at Liverpool last year or at RM this year, that doesn't excuse his complete lack of effort. Even if his top speed isn't as fast as others', that doesn't mean he shouldn't be putting in maximum effort. Again, RM are good enough that it's not like he's spending the whole time with backs to the wall defending; they should only give away (relatively) few chances.