That is according to Italian sports law expert and agent Filippo Pirisi.
The 29-year-old is one of the leaders of Paulo Fonseca's team after signing on loan this past summer, in a move that surprised many.
With United spoilt for choice at centre-back, Smalling was shipped out to the Italian capital.
The Englishman has taken to Serie A like a duck to water, having made nine appearances in the competition and two in the Europa League.
He told Tribalfootball.com: "In my opinion he will certainly be redeemed, he is showing all his value in a difficult championship as is the Serie A.
"I think that the ones who criticised the operation in the summer did not know well the player and today they must rethink the whole thing. I think he is, right now, one of the best expressions of the new Roma."
The Red Devils are reportedly looking for at least £17m for Smalling.