Morrison, 20, was drafted in by Town as cover a month ago following an injury to Antony Kay, but has failed to appear for Lee Clark's side as central defenders Peter Clarke and Jamie McCombe helped keep six clean sheets in seven matches.
Clark said: "I'm gutted to lose Sean because he's a very good footballer and has shown a terrific attitude all the time he's been here.
"I'd like to have given Sean some game time, but how could I when the two lads in possession of the shirts have been playing the way they have?
"It's a blow that he's gone back, but he's Reading's player and I respect their decision."
Source: PA
Source: PA