Fernandez, 20, spent last season on loan in the Championship with Preston North End, developing greatly, and will now look to do the same back in his native Spain.
Alvaro was part of the first team's pre-season preparations this summer and started against Leeds, Lyon, Wrexham and Athletic Bilbao. He provided the assist for Marc Jurado's goal against Wrexham in San Diego.
Fernandez was also among the substitutes in Sunday's 3-2 comeback victory over Nottingham Forest at Old Trafford.
Last term, Fernandez made a total of 42 appearances across all competitions for PNE, contributing six assists as he impressed in his first season in senior football.