Having starred alongside strike partner Alexandre Lacazette in the 3-1 comeback win in the first leg, Aubameyang ensured Unai Emery's side grabbed a scintillating victory away in Spain.
He scored three of the visitors' four goals in a 4-2 win on the night, after Valencia started brightly via Kevin Gameiro opener.
“We needed him and we needed Laca,” Monreal told club website.
“We are so proud of him because he’s scored three goals in a semi-final. He’s helped us a lot but it’s not only him, it’s every player.
“He scored a hat-trick, so we are all really happy for him.”
Arsenal will now face London rivals Chelsea in the final of the Europa League.
Aubameyang’s feat on Thursday sees him reach 29 goals in 49 appearances across all competitions, with eight in the Europea League.
He will hope to hit the 30-goal mark when they face Burnley on the Premier League final day.