The Gunners lost 3-1 to their rivals last Saturday, a result that left them 12 points behind Antonio Conte's men at the top of the Premier League.
Alonso put the hosts in front with a goal described after the match by Wenger as "a 100 per cent foul" and "dangerous play".
Wing-back Alonso had towered above Hector Bellerin and made contact with his face to head in, leaving the Gunners defender with a concussion.
And Wenger has now gone one step further by branding the goal "scandalous", saying it was vital in deciding what he suggested was an even match between the two sides.
"Overall we did not defend well, especially on the goals," Wenger told Sky Sports.
"It was a very even game with unexpected goals and chances. If you look at the saves made by the two goalkeepers, it was a very even game.
"On top of that, I think we made some tactical mistakes, especially for the first goal and on top of that, we were unlucky as the goal was a scandalous goal."