Mourinho was sacked by Manchester United in December in a move that cost the Premier League club £19.6 million in severance pay for the Portuguese and members of his staff.
"He's in the stands with (Lille sporting advisor) Luis Campos. He came to oversee Nicolas Pepe and Thiago Mendes," a source told AFP.
Campos is a friend of Mourinho's and served as chief scout when the latter was in charge at Real Madrid. Pepe, 23, is the third leading scorer in France with 16 goals, trailing only Paris Saint-Germain duo Kylian Mbappe and Neymar, and also has eight assists for second-placed Lille.
United replaced Mourinho with former striker Ole Gunnar Solskjaer halfway through his third season at Old Trafford with the club languishing in sixth and 19 points off the pace. He has pledged to return to "top-level football" with Real Madrid, the club he coached from 2010 to 2013, touted a possible landing spot for the 56-year-old.