The South American forward flourished at Emirates Stadium during his time working under an iconic coach, with his reputation firmly enhanced in north London.
He was, however, to take the decision in January to head for pastures new, as his contract with the Gunners ran down, and crossed the Premier League divide to end up at Old Trafford.
Things have not gone to plan for Sanchez in Manchester, who was considered to be quite a coup upon his arrival, with just four goals recorded so far.
So lost has the 29-year-old looked that he is generating more transfer talk 12 months on from his deal with the Red Devils, with Jose Mourinho struggling to bring the best out of him.
Wenger is saddened to see a former charge enduring such a dip, with the Frenchman telling beIN SPORTS: “I believe he has lost confidence.
“The strength of Alexis Sanchez is to take initiative to dribble and to take people on.
“Those are the players who are most vulnerable when they lose confidence their game is based on that and having that feeling to take initiative, he has lost that slowly.
“Since the start of the season he had a high level of physical energy but he has lost that as well.
“Certainly he is refreshed now because Mourinho left him out for a while.”
After grabbing a dramatic winner in a 3-2 victory over Newcastle prior to the October international break, Sanchez has been used sparingly since.
He was handed just five minutes off the bench in a 2-2 draw with Chelsea, before missing out on Champions League and Premier League meetings with Juventus and Everton.
Mourinho did welcome him back for a clash with Bournemouth, as Romelu Lukaku slipped out of the fold, and a starting berth was retained for a European return date with Juve.
United claimed 2-1 successes on the road in both of those fixtures, but Sanchez is still searching for his spark as awkward questions continue to be asked of his value to the cause.