I always try to just defer judgement on transfers until they've been here a year and assume Pep and co. know best but this transfer feels so left field.
We clearly invested in Trafford as a future no.1. GK is not like any other position, he can't get good enough to take over just by playing cup games. Bringing him in with Ederson or Ortega around for a year or 2 to take pressure off made tons of sense, bringing in Donnarumma completely upends that plan which was seemingly made a few months ago.
That's before you even take into account the fact Donnarumma will not accept missing CL games, he's a massive drama queen, and despite being an exceptional shot-stopper, he's not particularly good with his feet (Hence why Luis Enrique wants rid) and he's not the best in the air either despite being massive.
It looks like we're changing plans on the fly and setting ourselves up for drama, because Trafford would be well within his rights to ask to leave if he was brought in to be #1 (the club's plans for GK would surely have been part of the discussions when he was signed) and ends up behind a 26 year old mega-signing on 300k/week.
Consider also the prospect that Trafford and Donnarumma compete on a level playing field and Trafford is better - not impossible given his better footwork, familiarity with English football and obvious potential - then we're stuck with an unsellable backup keeper on huge wages who's causing drama because he's not in the team.