The Costa Rica international had previously struggled to make an impact in North London, prompting plenty of questions over his ability to feature predominately for the Gunners.
But amid an injury crisis he was thrust into the team, and Wenger cites his performance against Olympiakos as the moment he and everyone else was fully aware of his true capabilities.
“It was in the Olympiakos game when we had a knife on the throat that Joel turned up with something special," Wenger explained. "That’s when I thought ‘he can play well under pressure."
Campbell arrived at the club from Costa Rican side Saprissa as a promising youngster and spent the first three campaigns out on loan at various European clubs.
He returned to the Emirates in the summer of 2014 following a decent World Cup and many expected him to kick on. However, that was not the case and he was soon farmed out on loan again.
He struggled this time, at Villarreal, and many predicted that he would be sold last summer.
In a last-chance saloon sort of scenario, Campbell finally delivered in the absence of Aaron Ramsey, Alexis Sanchez and Alex Oxlade-Chamberlain and was instrumental in Greece.
Arsenal headed to Olympiakos knowing they had to win by a minimum of two goals, or by the odd goal but scoring three, and they did just that as a Campbell and Olivier Giroud inspired side won 3-0.
The 23-year-old has now racked up ten Premier League appearances this term, scoring twice.