The 21-year-old - out for almost 18 months with ankle and knee injuries before returning to action this season - capped a man-of-the-match display to finally beat inspired Swansea goalkeeper Michel Vorm with a 20-yard strike on 86 minutes to set up a trip to Brighton and keep alive hopes of Arsenal's first trophy since 2005.
"Jack was outstanding tonight," Wenger said. "He played in a different position, a bit higher up and it suited him well. I didn't expect him to be this good at this stage.
"I always thought that if we could get Jack back to a reasonable level by January we will have done well, but he is ahead of what I could imagine.
"He is back to what he was before the injury, I think so, yes.
"A complete midfielder can defend and attack and Jack can also dribble and give a final ball so the closer players like him are to the goal the better it is for them.
"He has quality and enthusiasm and a love for the game, which is the most important thing for me."