The 22-year-old has started 15 top flight matches for the Red Devils in the current campaign and the Scottish coach has hailed his growing maturity at right-back.
Sir Alex admitted that the Brazil international had given United some "scary moments" with his decision-making in the past but praised his recent progression.
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"The truly eye-catching improvement this season so far for me is the maturity that young Rafa has added to his game," Sir Alex told United Review.
"He has always shown himself as a defender with ability and great enthusiasm, but not always with great decision-making, especially in European football, giving him - and us - some scary moments.
"His performance against Queens Park Rangers [last month] was outstanding with his defensive work and attacking making his presence a key factor in our 3-1 win.
"I would have no hesitation saying that he is our most improved player this season with a couple of goals a reflection of his eagerness to lend a hand up front."