The English centre-back was on loan at AS Roma for this season, where he excelled.
However, the 30-year-old is back at United, as the two clubs could not agree on a permanent fee.
The Sun claims United want a fee of £20 million from any club who wants to sign Smalling this window.
Smalling wants the return to Italy and to play for Roma permanently, but the capital club may not have the money to match such a fee demand.
Inter Milan have also shown an interest in the England international.