2) This will not be Mustafi’s first time in England
Most of you probably won't know this but Mustafi was a part of Everton's academy before David Moyes lost faith in him and shipped him out to Sampdoria where he made a name for himself. Mustafi spent 3 years (from 2009 to 2012) at Everton but made just a single appearance for the club in their Europa league game against BATE Borisov.
One of the biggest challenges for a new arrival at the Premier League is adapting to the country, the culture, and lifestyle and the way the game is played. Mustafi, though, shouldn't have that problem and would be expected to settle in from the get go, unlike Gabriel.
The Brazilian who struggled to find his feet for at least half the season after arriving in January is still finding it extremely difficult to adjust to Premier League football and this is where Mustafi would be expected to have an advantage.