Being able to welcome Laurent Koscielny back after a minor injury is obviously good news for Arsenal.
However, given Koscielny's performances last season and Vermaelen's role as captain, Mertesacker seems certain to occupy the bench from now on. Yet is that really best for Arsenal?
The towering German international endured a tough debut campaign with Arsenal last season. Like many before him, Mertesacker had his problems adapting to the frenetic pace and rugged intensity of the English Premier League.
Despite many fans unfairly targeting the ex-Werder Bremen skipper as a symbol of the Gunners' defensive failings, Mertesacker improved considerably around the turn of the year. Unfortunately, a season-ending injury struck against Sunderland in late February, just as Mertesacker was approaching his best form for Arsenal.
In his absence, Thomas Vermaelen partnered with Koscielny to become the featured central defensive pairing for the Gunners. Koscielny was solid all season, significantly improving on his first year in England.
Much in the same manner, Mertesacker has begun his second season in the EPL looking more composed and accomplished. He has played a key role in Arsenal keeping two clean sheets to begin the campaign.
That's no slight on the two players, but rather an indictment of how poorly they complement one another at the heart of Arsenal's defense. The problem is they are just too similar.
It's not a similarity in style or personality. Vermaelen's determination, buccaneering style and knack for a goal make the Belgian an easy player to like. However, those same attributes also mean Vermaelen is not always the most reliable defender.
He will take chances going for the ball and his enthusiastic forays forward can leave Arsenal exposed. Koscielny is less naturally reckless and a more precise and technically sound tackler.
However, both players love to step out to attack the ball. They are both defenders who believe their anticipation and athleticism will often allow them to intercept or simply break up possession.
The problem is that their similarities can cancel each other out and leave huge spaces in the middle of defense. The nadir for their inconsistent partnership came in last season's 3-3 home draw against Norwich City, where the Canaries routinely caught out the advancing pair and exploited the middle.
The fact is the best central defensive partnerships feature a contrast in styles. Sol Campbell and Kolo Toure were so effective because Campbell offered the physical power and reading of the game that allowed Toure to use his pace and step out and win the ball.
He may not be particularly aggressive or rugged, but Mertesacker's 6'6" height can always be useful in the EPL. An example was his comfortable handling of a regular Arsenal aerial tormentor, Peter Crouch.
Another point in Mertesacker's favour is his noted intelligent reading of the game. He is a positionally sound, stay-at-home defender.
An absence of pace and limited natural agility will always count against Mertesacker and have to rate as a real concern in the frantic environment of the EPL. However, combing Mertesacker's patience with the active style exemplified by Koscielny or Vermaelen would give Arsenal a more suitable combination at the heart of its defense.
Despite the high-profile reputations of Koscielny and Vermaelen, Wenger at least ought to find out how long Mertesacker's solid form can last.