Just two minutes after the Gunners fell behind to Pizzi's penalty, Bukayo Saka scored an away goal from close range to secure a draw that gives Arsenal the edge of the last-32 tie.
Bruno Fernandes and Marcus Rashford are in the line-up for the last-32 first leg, which takes place in Italy. Adnan Januzaj, once of United, and Man City legend David Silva start for La Real.
Cristiano Ronaldo goes back to Portugal as Juventus take on Porto, while Dortmund face Europa League winners Sevilla. Meanwhile, in the Premier League, leaders Man City visit Everton.