#3 Dani Ceballos allows frustration to get the better of him
Dani Ceballos started the game at the heart of Arsenal's midfield but a superb display by Dani Parejo and Etienne Capoue meant the hosts dominated proceedings from the middle.
This saw Ceballos and his midfield partner Thomas Partey frustrated early on, but it was the Real Madrid loanee who lost his cool.
A series of rash tackles were made before his booking in the 44th minute, but rather than towing a safer path in the second half, the 24-year-old continued to live life on the edges and he was deservedly sent off for a stamp on Parejo.
His midfield partner Partey was also shown a yellow card for protesting the decision and Ceballos' dismissal meant that Arsenal played the final 33 minutes of the game with just 10 men.
#2 Balanced scoreline means there is all to play for in the second leg
As already indicated, Arsenal's away goal gives them a massive lifeline heading into the second leg, but Villarreal will still fancy their chances against a Gunners side that have been plagued by inconsistencies.
Villarreal themselves have not exactly set the world alight with their displays this season and the tie is still very much in the balance.
The defeat means Arsenal must secure a win on home soil next week, while Villarreal just have to avoid losing at the Emirates to progress to the final.
Both sides have what it takes to get the job done in the second leg and the finely balanced scoreline means that another exciting game could be on the cards in the English capital.
#1 Impressive Bukayo Saka steps up when it matters most for Arsenal
Bukayo Saka has arguably been a revelation for Arsenal and having had his breakout campaign last term, he has continued his impressive trajectory in the current season.
With Pierre-Emerick Aubameyang and Alexandre Lacazette missing through injury, the onus was on the 19-year-old to lead the line and he did so with minimal fuss.
Even as Arsenal struggled to clear their lines, the England international continued to charge forward and put Villarreal's defense on the backfoot.
His efforts reaped dividends when he won the penalty from which the visitors scored. He also drew the foul from Capoue that got him sent off and saw Arsenal play the final 15 minutes at numerical par with their hosts.
