With 36 year-old Irish star Duff ruled out for the campaign after ‘rupturing a tendon in his calf and suffering damage to his ankle ligaments’ last month, City moved quickly to add 23 year-old Novillo as it pursues a finals spot.
Novillo - a product of the Lyon youth system - can play on either wing and joined his new team-mates in training on Tuesday morning at the City Football Academy after his arrival in Melbourne on Monday night.
"Harry has proven experience playing at a high level as a winger in France and we are confident that he will help us for the remainder of the A-League season," Melbourne City coach John van't Schip said.
After making his Ligue 1 debut in 2010 for Lyon, Novillo has predominantly spent his career in the French second tier, before a move to Mons in Belgium in 2013 and a return to France with Clermont.
Pending international clearance, Novillo will be available for City's A-League round 20 meeting with Central Coast Mariners at Central Coast Stadium on Sunday.
