To address that hypothetical, this article grades Mensah's performance against Germany at the 2014 FIFA World Cup and provides a scouting report on his season for Evian.
Mensah vs. Germany Grade: A
Starting as a left-sided centre-back alongside John Boye, Mensah was immense in dealing with German right-forward Mesut Ozil interchanging from the flanks to a central position.
Mensah made two tackles, cut off three passes and cleared the ball 11 times, per Squawka.com.
Mensah was an important factor in Ghana getting a crucial point in their 2-2 draw against Germany.
Furthermore, Mensah was a passing outlet from the back.
The reason why Mensah did not receive an A+ grade was because Germany scored twice, and he was partially culpable when German substitute Miroslav Klose poached a goal.
Mensah Evian Positives
Mensah is a tactically flexible defender, who can start at right-back, centre-back and left-back.
Averaging 0.7 fouls per game, Mensah is a conservative tackler and is safer compared to then-United (now Inter Milan) centre-back Nemanja Vidic, who averaged 1.3 fouls per game.
Mobile and rangy, Mensah has a spring in his step.
He is excellent in the air and can replicate Vidic's aerial prowess.
Mensah covers the ground swiftly, and he is at his best intercepting and clearing the ball, like cutting off a cross zoned in at Rennes' on-loan Benfica striker, Nelson Oliveira.
Mensah Evian Negatives
There are elements of naivety in Mensah's game.
Upon muffing a clearance, Mensah inadvertently plays in Saint-Etienne right attacking midfielder Romain Hamouma.
Mensah miscues his header, which falls into the path of Nice substitute Alexy Bosetti.
After barging into Lyon centre-forward Bafetimbi Gomis, Mensah receives a yellow card from referee Tony Chapron, only to turn it into a red card for dissent.
Mensah needs to be more consistent if he has desires to play for a major European club.
There are shades of Chris Smalling in Mensah in that, while his potential is great, the regular misjudgements are a red flag.
That said, Mensah's upside is high, and he has the quality to fill Vidic's void at United.
Mensah was impressive against Germany, and he is a defender to watch out for. By the next World Cup, he could be a world-class defender.
Statistics via WhoScored.com