Di Maria joined United from Real Madrid in 2014 for £59.7million – a then-record British transfer fee.
The Argentine was reluctant to leave the Spanish capital, however, and after a bright start he struggled to find his footing in Manchester under Louis van Gaal.
Fans began to turn on the winger who left the following summer for £44.3m after four goals and 12 assists in 32 appearances.
Di Maria showed no love was lost last term when United met his current club Paris Saint-Germain in the Champions League, appearing to swear at United fans while celebrating one of PSG’s goals at Old Trafford.
Smalling, who is currently on a season-long loan at Roma, has gone against the tide with his opinion on the 31-year-old, though.
“This might surprise you but Angel Di Maria was one of best players I’ve ever seen,” the defender told The Athletic.
“It ended sourly, but when he first signed he looked incredible.
“He did it on the field at the start too, he chipped a goal at Leicester and I thought he would take us to the next level.
“All of us watched him and thought: ‘We just need to get him the ball and he’ll do something special.’
“It’s a shame it ended for him so quickly at Old Trafford, that he has been considered a flop because he’s a great player.”
Di Maria quickly rediscovered his form at PSG and has scored 76 goals and assisted 77 in 195 appearances for the Ligue 1 club.