Louis van Gaal may not sign any more new players after being impressed with Manchester United’s pre-season form.
Reading are keeping tabs on Leyton Orient defender Elliot Omozusi, Sky Sports understands.
The 25-year-old was one of the mainstays of Southampton's promotion campaign last season, but he has found first-team opportunites limited since the Saints entered the top flight.
The Belgian has been restricted to just seven appearances in all competitions this season, although he has featured in four games since the turn of the year.
De Ridder, who spent three years with Dutch side De Graafschap before moving to Southampton in the summer of 2011, becomes Dougie Freedman's fifth signing of the transfer window.