Strasbourg 2 Monaco 1: Sy howler and Grandsir red card cost Henry in first match
Henry replaced Leonardo Jardim seven days ago after Monaco picked up just six points from their opening nine matches, but their display in Alsace showed no signs of an instant improvement despite the former France star's arrival.
It took the home side 17 minutes to move ahead, but they relied on a woeful error from Sy, who allowed Adrien Thomasson's header to squirm out of his arms and into the net despite it looking like a routine save.
The visitors then lost Radamel Falcao to a muscular injury just before half-time.
But Monaco's biggest blow came in the 66th minute when Grandsir – introduced as a substitute just one minute and 51 seconds earlier – was shown a straight red card for a high foot on Anthony Goncalves.
2015 -Samul Grandsir is the fastest substitute to get sent off in Ligue 1 (1 minute 51 seconds) since Benjamin Mendy (47 seconds) in September 2015 with Marseille v Toulouse. Jinx. pic.twitter.com/e2U7nImVVM
— OptaJean (@OptaJean) October 20, 2018
Lebo Mothiba effectively wrapped things up six minutes from time, racing clear and chipping a clever finish over the approaching Sy.
Youri Tielemans pulled one back from the spot in second-half stoppage-time after Moussa Sylla – Falcao's replacement – was tripped, but it only proved to be a consolation, leaving Monaco one place off the bottom after their second worst start to a Ligue 1 season after 10 matches.
1953 - Monaco have picked up only 6 points after 10 Ligue 1 games this season, their 2nd lowest tally at this stage ever, after 1953/54 (5, with 3 points for a win). Hell. pic.twitter.com/TnSssFn3GN
— OptaJean (@OptaJean) October 20, 2018