In hindsight Maguire should shoulder some of the blame, as he called for the ball in a terrible position and put De Gea under pressure in the play before. But De Gea lacks all awareness and composure required to make the right call. He had other options, but once the ball came to Maguire there was no saving it. Maybe Martinez would've done better than Maguire, but I'd argue Heaton would've done better than De Gea here. Just imagine Edwin van der Sar instead of De Gea, and the difference in presence is massive. It's not that Maguire or De Gea are bad players, not at all. They just lack the skill set to play out from the back. Recently the constant presence of players like Shaw, Varane or Martinez compensated for their shortcomings, but now with all these injuries they have to sort it out themselves and it's clear they cannot.
As a sidenote, Casemiro looks suspect as well, playing out from the back. Might as well switch out Eriksen for Fred and bring in Rashford for Antony for a good old round of oleball. Nothing wrong with playing the long ball if that's what the players are best suited to.