The Saudi Pro League side dismissed him from his role on Wednesday after winning just six of their opening 12 league games this season.
After a 2-0 AFC Champions League defeat to Al Quwa Al Jawiya, Nuno and Benzema reportedly exchanged insults in the changing room.
Al Ittihad then announced that they had ended the Portuguese coach's contract after he spent over a year at the helm.
The club returned to action on Friday when they hosted Abha Club in the Saudi Pro League, with Benzema the star of the show in a 4-2 win.
His first goal of the night came on the approach to half-time when he scored a penalty, converting it down the middle of the goal.
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Karl Toko Ekambi drew Abha level before Benzema turned provider for Igor Coronado to put Al Ittihad back in front.
The former Real Madrid star then scored two goals in three minutes to complete his hat-trick.
A 67th minute effort into the roof of the net was followed by a low strike in the 69th minute, with both finishes coming on the counter.
Fahad Al Jumayah netted in stoppage-time for Abha but it was just a consolation goal as Al Ittihad won their first match without Nuno.
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The former Tottenham boss joined in July 2022 and led them to the Saudi Pro League title in his first season.
He also lost just seven of his 53 matches as boss but Al Ittihad opted to make a change, with Hassan Khalifa in temporary charge.
Benzema has now taken his goal tally to nine in 13 appearances for the club since he signed in the summer after his Madrid departure.
All of his strikes have come in the Saudi Pro League, with the striker yet to net in the King's Cup or the AFC Champions League.