The Gunners manager made only one major off season signing, that of former Chelsea goalkeeper Cech (how the Blues could do with him now), for $22 million.
It's a considerable sum for a goalkeeper, but one many judges considered a game-changer. This was the signing that would transform Arsenal from bridesmaid, and take it to the altar - no need to bring up the failed bid to sign a top-class striker yet again.
Eleven trophy-laden seasons for Cech at Chelsea brought four Premier Leagues, four FA Cups, three League Cups, two Community Shields, one UEFA Europa League, and on that glorious night in Munich, one UEFA Champions League.
Cech oozes experience, composure, and class. Immediately, with Cech in goal, Arsenal added this season's Community Shield to its trophy cabinet, when Wenger finally beat Jose Mourinho.
So, where was Cech against Olympiacos?
Ospina, who proved himself capable - but certainly not world class - in his stint as Arsenal's first choice in the second half of last season, deputised.
And he dropped the ball over the line from a corner.
It cost Arsenal dearly.
Ospina also played in the first match against Dinamo Zagreb, which the Gunners lost, while Cech cooled his heels in both UEFA Champions Leagues matches, and in the League Cup tie against Tottenham Hotspur.
The League Cup is the place for rotating goalkeepers, and giving the second choice a chance to keep sharp with some game time.
The UEFA Champions League, club football's holy grail, is not.
Wenger said Cech needed a rest, and that Ospina needed game time.
The latter may be true but if Cech wasn't signed for Arsenal's big European nights, then what was he signed for at all?
Cech played 16 matches in all competitions for Chelsea last season. Even at 33, he is fresh.
Seven Premier League matches so far for the Gunners shouldn't preclude him from playing against Olympiacos.
In fact, the decision not to play him, especially after the lost first up to Zagreb, reeks of the bloody mindedness and stubbornness for which Wenger is fast becoming renowned.
Wenger said after the game that we couldn't be sure Cech wouldn't have made the same mistake.
But, we can take a pretty good guess.
Stubborn Wenger has cost Arsenal again.
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