The La Liga champions have named four skippers for the new campaign, along with Sergio Busquets, Gerard Pique and Sergi Roberto.
Messi was named club vice-captain in 2015 and, having also captained Argentina since 2010, was the obvious choice to take over from Iniesta.
All four players came through La Masia which means the team will be captained by a player who came through the ranks in each fixture.
La Liga and the Super Cup only allow three non-EU players to be included in any matchday squad and Coutinho is registered with a Brazilian passport.
New signings Arturo Vidal of Chile and Brazilian pair Malcom and Arthur Melo all need to be on the non-EU visas, so the 25-year-old is set to switch to Portuguese registration.
Coutinho’s wife is Portuguese and the player is able to register as a Portuguese citizen after three years of marriage.
The Portuguese captain joined Juventus last month in a staggering €117-million deal after nine seasons in Madrid.
During his stint, the 33-year-old won four Champions League titles as well as two La Liga crowns in a trophy-laden spell.
But Ter Stegen insists their fiercest rivals will be a serious threat when the new season begins.
“I think they have an amazing team, like every year, just like us,” he said. “It’ll be equally difficult to play against them with or without Ronaldo.
“You have to understand that this is a player who made the difference in a lot of games, but we focus ourselves on our game and we know that, if we’re confident, we have the chance to do it all.”