The Gunners were down 2-1 before Tierney equalised with a brilliant solo effort, and eventually won the match 3-2 thanks to a late header from Pierre-Emerick Aubameyang.
And Keown thinks Tierney helped inspire his teammates to fight for the win.
He told BT Sport: "It is the symbolism you want. That's the player you want, someone who is in charge of your own destiny and he proved it there.
"But he has to keep doing that, we have to keep him fit and have to keep this momentum going."