Mertesacker replaces Mikel Arteta, who retired as a player when his contract expired in June and has joined Manchester City's coaching staff under fellow Spaniard Pep Guardiola.
Messi's future at Barcelona is uncertain after the Argentine captain surprised Barcelona hierarchy on Tuesday night when he asked for his contract, which has one year still to run, to be terminated under an end-of-season release clause.
Pogba's performance was subject to criticism with former Man United captain, Gary Neville launching a fierce rant at the 26-year-old. "Pogba is now playing a little bit as he did pre-Solskjaer under Jose Mourinho," Neville told beinSPORTS.
The 35-year-old actually won the league title three times at United, including the last time the side won the league under Sir Alex Ferguson 10 years ago.
It's the captain, it's the captain of the most successful team in Europe in the last years, it changes a lot. "We cannot lose our heads about this decision.
Manchester United captain, Harry Maguire, has been told that he could beat jail by paying £90,000, according to the UK Mirror. Maguire left a Greek court yesterday wearing a face mask and to a warning he could spend three years in prison.
Arteta, who played under Wenger at Arsenal between 2011 to 2016, and went on to captain the club, thanked the Frenchman for helping his dream come true. The 37-year-old stated this during his official unveiling on Friday.