The man tapped as Cuban President Raul Castro's chief lieutenant and likely successor must quietly fend off any challenges from within the Communist-run island's secretive citadel of power.
He must gain legitimacy with young, and even middle-aged, Cubans who have never known a leader not named Castro. And he must deal with an exiled diaspora and American officials who were already making clear on Monday they will not be mollified by a new, younger face.
"There's going to be a huge charisma deficit," said Ann Louise Bardach, author of "Without Fidel: A Death Foretold in Miami, Havana and Washington." "You go from Fidel to Raul who at least had some of the shine of the Castro mantle, somebody who fought in the revolution."
She said Cuba faces "massive" problems including a large public debt, dependence on Venezuela, an aging population, decades of brain drain and one of the world's slowest Internet connections.
Whether Diaz-Canel is the man to fix all that is very much open to debate. Will Cubans accept another leader who was hand-picked from above and whose ascension—if it happens—will not come through multiparty democratic elections?
And will those passed over for the top job fall in line? If Fidel and Raul Castro are still alive, will the 52-year-old electrical engineer and former minister of higher education be able to set his own course?
On the streets of Havana, a day after Castro accepted a new term and said he would step down when it ends, many had their doubts about the future.
"Change? Was there a change?" asked Ernesto Silva, a 25-year-old student who scoffed at the idea that the country must wait another five years for new leadership, and said he hopes to emigrate to the United States in the meantime. "I find it hard to believe that he will be able to do, say or truly change anything."
Others were more enthusiastic, but still unsure how Diaz-Canel will establish control.
"I think it is good. He is a new and young figure. But he was trained by the old guys," said Maria Quesada, a 45-year-old office worker. "We still can't talk about a government without Castro because he's still going to be there, and I think the true test for Diaz-Canel will be when his vision differs from Raul's."
Raul himself faced a similar challenge when he took over from the ailing Fidel in 2006. The younger Castro was seen as a bland and unassuming figure who had always operated in the shadow of his larger-than-life older brother.
But he has overseen a series of sweeping changes since then in an effort to right the country's ever-weak Marxist economy, expanding private enterprise, legalizing a real estate market and eliminating most travel restrictions.
Behind the scenes, Raul has led an anti-corruption campaign and replaced many of Fidel's confidantes with loyal military officials who earned his trust during his four plus decades as the nation's armed forces chief.
Observers say it is those men, who have been put in charge of important state-owned enterprises like the phone company, the enormous holding company Cimex and virtually the entire tourism industry, who Diaz-Canel must persuade to follow him.
"I'm sure he's shown himself to be acceptable to the military already, otherwise this would never have happened," said Paul Webster Hare, the British ambassador to Cuba from 2001 to 2004 and now a lecturer in international relations at Boston University. "He has to be acceptable to them."
In Washington on Monday, State Department spokesman Patrick Ventrell said the U.S. remained "hopeful for the day that the Cuban people get democracy, when they can have the opportunity to freely pick their own leaders."
"We're clearly not there yet," he added.
Ventrell said Cuba needed to do more to open up if it wants repaired relations with the United States.
Still, observers said the naming of a successor, along with the economic and social changes Castro has instituted, could eventually lead to detente, or at least an easing of bad blood.
"There is a psychological and political benefit to naming somebody now," said Julia Sweig, a Cuba expert at the Washington-based Council on Foreign Relations. "It can demonstrate to the United States that there is an opportunity to change how we relate to this new Cuba."
Reaction to Diaz-Canel's promotion was met with a shrug by Cuban-American politicians who said that changing the nameplate outside the presidential office won't alter a system they see as repressive and undemocratic. And some analysts and exiles in South Florida questioned whether he will survive politically long enough to ever take control.
Little is known of Diaz-Canel's relationship with Cuba's other political or military elites, or his own personal ideology.
He is said to be a fan of the Beatles who in his youth wore his hair long at a time when both acts were considered anti-revolutionary. And a former colleague says he is a private jokester despite his somber public demeanor.
Like almost all Cuban politicians who earned their stripes by mastering the art of backroom party maneuvering, Diaz-Canel seems to have learned long ago that there was no benefit to putting himself in front of the cameras.
It was a lesson he probably learned back in 2002, when fellow young turk Roberto Robaina was dumped as foreign minister after being accused of "political and ethical errors."
Diaz-Canel, who was part of Robaina's circle, not only survived the purge and a subsequent one that took down the next foreign minister and a prominent young vice president—but he appears to have thrived behind the scenes.
Robaina, who now paints pictures and runs a private restaurant in Havana, would not comment Monday on Diaz-Canel's rise. But a former colleague who has known Diaz-Canel since the 1980s said his career really began to take off after the purge.
In 2003, he was named head of the Communist Party in Holguin, a role he had held previously in his native Santa Clara. The same year, Raul Castro helped get him a seat on the Political Bureau, the island's equivalent of a Politburo.
The experience in Holguin was a trial by fire for Diaz-Canel, who had been popular in Santa Clara but met resistance in his new role.
"People didn't like him in Holguin," said the former colleague, who would speak only on condition of anonymity because of the sensitivity of the matter. But he caught Castro's eye, and earned his praise.
"He has demonstrated a solid ideological firmness," Castro, who was then vice president himself, said of his protege, perhaps presaging his future rise.
Cuban officials insist that a post-Castro era will be no different and that the nation's institutions will unquestionably back the new prince. They point to a long history of exiles wrongly predicting the collapse of the revolution, which has so far survived the demise of the Soviet Union and retirement of Fidel Castro, not to mention Washington's 51-year economic embargo.
But Hare, the former ambassador, says it would be a mistake to underestimate the challenges Diaz-Canel would face getting state machinery and political rivals to fall in line.
"Does he have rivals in the 50-something generation in Cuba who perhaps resent what happened?" Hare asked. "There is a scenario where other people will be sensing ... that Diaz-Canel doesn't have all the attributes of the Castros and therefore (will ask) 'Why shouldn't I have a chance at it?'"
Peter Orsi and Anne-Marie Garcia in Havana, Bradley Klapper in Washington and Christine Armario in Miami contributed.
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