England international Smalling joined Roma on transfer deadline day on a four-year deal worth £18million.
The 30-year-old spent the 2019-20 campaign on loan at the Italian club, helping them finish fifth in Serie A and qualify for the Europa League.
Smalling spent a decade at Old Trafford, making 323 appearances for the club and winning two Premier League titles.
Jamie Redknapp has praised Smalling and says he is better than current Manchester United defenders Victor Lindelof and Eric Bailly.
His dad, the former Tottenham manager Harry Redknapp, also believes Manchester United have made a mistake by selling Smalling.
Jamie Redknapp told Sky Sports: ‘If you looked at Manchester United on Sunday, you could be getting a player in for every position.
‘Not one of those players did anything or showed anything that they want to be a Manchester United player. The players he has have got to do so much better.
‘‘It’s not easy to get players in at the highest level, the Virgil Van Dijks don’t come along very often.
‘But Manchester United need to be buying top-level players. That yesterday was as bad as it gets.’
Harry Redknapp added: ‘You look at Manchester United, I would think him [Smalling] and Maguire would be their best partnership.
‘Now, suddenly he’s going. They’re very short in that area. They’re very open at the back. I would have kept him, I wouldn’t have sold him.’