Maguire, who had also been linked with West Ham, decided to join Leicester City during the week.
Smalling has been linked with a move away from Manchester United after they announced the signing of Swedish defender Victor Lindelof from Benfica.
The England international had already fallen down the pecking order at Old Trafford, with Jose Mourinho preferring summer signing Eric Bailly.
Manchester United are believed to value the defender at around £15 million, with previous reports suggesting West Ham were readying a move worth around £10 million.
With the World Cup just around the corner, Smalling may have one eye on the tournament and go in search of regular first-team football elsewhere.