The 19-year-old has scooped back-to-back monthly awards for the first time in his United career, having also claimed the accolade for his performances in November.
Alejandro scored two goals and registered two assists across the month, which included a Man-of-the-Match display against Aston Villa in our comeback victory at Old Trafford on Boxing Day.
Garnacho's brace preceded Rasmus Hojlund's winner, as the Reds came out on top in a five-goal thriller, after going in two down at half-time.
Alejandro garnered a whopping 64 per cent of the vote, beating fellow Academy graduates Jonny Evans and Kobbie Mainoo to December's prize.