In principle? There is no principle here, Marotta is simply prepping you mentally for Skriniar's departure.
In principle, it's always the quality and mentality that's hard to replace, not the position. Look at Liverpool, do you think they will find more difficulty replacing Salah than van Dijk? In fact, they have been struggling a lot to find an adequate partner for van Dijk while renewing their attack just fine with Jota, Diaz (and Darwin, but too early to tell).
Maybe Bremer presents a unique opportunity and we're also desperate for cash but let's not kid ourselves thinking a defender is easier to replace than an attacker.