Last year for isntance they didn't have him for a long stretch and the team was playing really great, so he can be "replaced", yet not with another Trent.
Trent sometimes can be a double edge sword, when he is on his more Brunesque days trying the ultimate hollywood ball too much, when he tends to drift without a more clever pattern (and sometimes this neglects some defesive task asigned), he can be disruptive not on the best way (as he can be of course in the good side of the coin).
Liverpool BTW can try to seek for someone else mostly purely focus in the offensive/control department (with some dribbling/passing and more pause in him, it's a pitty for them that they could never have Alcantara filling that role due to his injuries) to add an extra quote of creativity these days with Macca coming from so deep with many defensive tasks involved and Slobo/Curtis/Trent/Diaz, etc also being players prone to the momentum, to the flashy attempt than those combine with a more methodic approach, that anyway Arne demands from them, they rarely loose their shape thse days like with Jurgen.
Arne has made them quite a more pragmatic block and Trent has been a lot less expose if he tries something fancy and that's great to relief him from an unnecessary burden and helps him to show his best side more often, they are a ver compact team, with many polivalent fellas.