The La Liga club have the option to make the move permanent for the 32-year-old Uruguayan in the summer.
"Diego has set an example," manager Carlo Ancelotti told PSG's official website.
"I want to emphasise that he was very professional. This is a good lesson for the whole group because he worked every day with great professionalism."
Lugano, who has not played for PSG this season, arrived at the Parc des Princes from Fenerbahce in summer 2011 for around £2.5m.