The two girls made a 5,600km journey to Cologne where they were sheltered as refugees. After discovering that Nujeen was a fan of Barcelona FC - and had never attended a game - the club surprised her by arranging a visit to Spain and watch them play Celta Vigo.
Prior to her departure from Cologne, Mustafa’s grandmother wished her a safe journey. Mustafa told her not to worry because she is "an expert on long walks."
Mustafa, who has cerebral palsy, has used a wheelchair since she was a child and for her trek from Syria to Europe that included crossing seas on a raft.
"What is the Barcelona bus doing here? Oh, my god!" exclaimed Mustafa as the bus pulled up to her house.
"Am I sitting in Messi's place?" she asked aboard the bus.
FC Barcelona documented Mustafa’s 18-hour ride to Camp Nou, the club’s stadium.
"Hey Nujeen!", Messi tells her in a video recording shown on the bus. "We know about your story and think it's incredible."
"Messi has a kind of a baby face, he looks younger even though he's 30, so it's like 'you have grown up now' because I know him since 2007," Nujeen told Reuters in an interview.
"At that time he looked like a baby," she added, before addressing the five-times Ballon d'Or winner: "You have changed, you look like an adult now."
"He's a really shy guy, that hasn't changed," said Nujeen, who hopes Barcelona win six titles again to match their feat in 2009 when they won the King's Cup, La Liga, the Champions League, the Spanish Super Cup, the European Super Cup and the Club World Cup.
Nujeen, who speaks Arabic, French, English and German, has become a symbol of the United Nations refugee agency (UNHCR) and hopes peace will someday return to her country.
Mustafa, who earned English from watching soap operas, had a book published in 2016 titled, Nujeen: One Girl's Incredible Journey from War-Torn Syria in a Wheelchair.
Many people only remember Syria as a war place, she explained during her trip to Barcelona.
"Through writing the book, it was important for me ... to tell people that I was not just a number. I'm a person. I'm not a report you see on the news,” Mustafa said.
At the stadium, Mustafa gave the club's president, Josep Maria Bartomeu, a copy of her book. In exchange, he gave her his own Barça lapel.
"I think the saddest part for me was that part of my life was now gone. No one will ever know how it looked like before the war,” she added.
"Sometimes I feel homesick, I ask myself what am I doing here, I want to go back home where I lived and loved you know, but I want to move forward I don't like being a victim, I am a survivor.
"I dream of becoming a physicist and perhaps an astronaut and I dream of... yeah one of my dreams is to meet the Queen of England and I still want to."
Recently a public debate on refugees, particularly immigration from the Middle East has raged in Germany and Europe.
Barcelona player Gerard Pique meets Nujeen Mustafa. Source: FC Barcelona
Mustafa said: "I understand you might fear us because of the fear of the unknown, because that is a natural human tendency... to fear that which you don't understand."
Gerard Piqué, one of the team's players, spoke with Mustafa after the game and gave her a Barça jersey with 'Nujeen' embroidered on the back.
"It's a story of overcoming adversity," said the Spanish defender, “and that's why in the dressing room all the players know about her story."
"Nujeen, thanks a lot for sharing your dreams with us," he added.
