Schweinsteiger, who had been training with the club's reserves according to media reports, was pictured alongside his team mates as Mourinho supervised a session on Monday.
The former Bayern Munich midfielder said in August that he would not join another club in Europe.
He played over 500 matches for Bayern, winning eight Bundesliga titles and the 2013 Champions League.
United, who are eighth in the Premier League standings, travel to Fenerbahce in the Europa League on Thursday, before a trip to 19th-placed Swansea City on Sunday.
(Reporting by Nivedita Shankar in Bengaluru, editing by Ed Osmond)
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