Trossard red card was not unfortunate, it's an obvious offense that deserves red card knowingfully well that he's on yellow already. Kicking the ball away after the refree has stopped the play is a cautionable offense and every professional player is well aware of this.
It's high time Arterta instill discipline on this players because this same offense led to Rice's red card against Brighton and Trossard repeat the same thing today because of Indiscipline and put the team on pressure for the entire second half. It doesn't make any sense committing unnecessary offenses when you are aware of the rules.
Arsenal played a fantastic game thou. I applaud how they adapt to the circumstances and share the point with the champions.