I gotta say I've no clue how good Fernandez actually is because I have seen him play exactly zero minutes but just looking at their career paths up until now I would say Maatsen is at least a year ahead in his development curve compared to Fernandez.
Maatsen started out his loan route in L1 at Charlton three years ago, then proceeded to play a full Championship season for a mid-table side at Coventry City before finally going to a top Championship club in Burnley and performing really well for a side who absolutely stomped the league. That's three seasons worth of loans where he has stepped up a level in club quality every year and hasn't looked out of place at all.
Fernandez' loan season at Preston would be comparable to Maatsen at Coventry in the 21/22 season. Following his Coventry loan last summer there were very few people around Chelsea who thought Maatsen was ready to take on a first team role and because of that we ended up signing Cucurella instead, but now it's a whole different situation. So maybe for United it makes sense to sign a loanee like Cucurella for this season and seeing if Fernandez can make a similar step up with another loan before looking back at it again next year. If he's looked good during another loan, you could just ditch Cucurella and integrate Fernandez into the first team for the 24/25 season.