Smalling had hoped to round off his loan with Roma by playing for them in the Europa League but an extension has not been reached and he is no longer with the Italian capital outfit.
"I think Chris has shown over his career at Man United how valuable he is for Man United," Solskjaer said.
"This season was about him being a regular, going to Roma, showing and proving how good a centre-back he is.
"Now he's coming back and Chris has shown this season that he will be valuable.
"I've been speaking to Chris throughout and been delighted with his season."