The 32-year-old has not featured since Mourinho's pre-season arrival and was written off as an asset by the club
The 32-year-old Schweinsteiger has not featured since Mourinho's pre-season arrival and was written off as an asset in the club's financial results for the year ending June 30, 2016.
The World Cup winner was also omitted from United's Europa League squad meaning he is inelgible for Thursday's match against Fenerbahce.
But Mourinho said Thursday that the player's experience could be key if the squad is hit by injuries.
"It's a decision based on the fact that we are in a period of the season where it's really difficult to train with everyone together," Mourinho told the club's official website in Istanbul.
"Looking at Bastian and the way he was working professionally with the fitness coach, we think that it's a human decision, a professional decision, to bring him back to the team.
"He'll be much better prepared and in much better condition in case his future decision is to leave the club; he'll be much better prepared for competition if he stays with us.
"It becomes one more option. At the moment, we have no problems in midfield like we have in defence but, if we have problems in midfield, it's another option for us. It's a human decision based on the training numbers and methods, but also a decision that can be positive for the group. It's positive for us and for him."
Schweinsteiger joined United from Bayern Munich before the start of last season when former manager Louis van Gaal was still in charge at Old Trafford.