The Gunners launched a dramatic deadline day swoop for the Monaco ace with a £92m club-record offer.
It was accepted but the player snubbed their advances and decided to stay at the principality club.
The news came as a big blow to Arsene Wenger as he tries to prepare for life after the wantaway Alexis Sanchez.
And Daily Record reporter Castles thinks Arsenal's window was a shambles and many areas of their squad are still in disarray.
Castles told the Transfer Window Podcast: "Arsenal had a terrible transfer window.
"A really shocking transfer window.
"Only one signing of significance, and one who in [Alexandre] Lacazette, for all his skills and finishing, is not a complete striker.
"Not a good aerial striker.
"Arsenal, in recent seasons, have moved towards an aerial game.
"Their defence is a complete mess, [it] desperately needed strengthening and nothing done there apart from [Sead] Kolasinac who is only a partial solution.
"And then the horrendous mess they made of trying to sign Thomas Lemar on deadline day, putting in a club-record bid when they didn't have a guarantee that Lemar would come to the club.
"Not only did they fail to get the player but they embarrassed themselves publicly in their attempt to sign him."
