Lyon signed Fernando Marcal on Thursday but lost a promising France U20 international to the reigning Ligue 1 champions one day later
Jordy Gaspar has left Lyon for Monaco, signing a three-year deal with the Ligue 1 champions.
Defender Gaspar, a France Under-20 international, made just three appearances for Lyon last season, one of which came in the Champions League.
The 20-year-old joined Lyon's youth ranks in 2008 and spent three campaigns with the B team before stepping up last season.
Monaco vice-president and chief executive Vadim Vasilyev said: "We are pleased to announce the signing of Jordy Gaspar to AS Monaco.