The gazelle-like 21-year-old, who is 1.74 metres tall and weighs 64.5 kilograms, has signed a five-year contract with the Spanish champions, who paid 57 million euros for his services.
After competing for Brazil on home soil at the Confederations Cup, he is likely to make his Barca debut in a friendly in Gdansk, Poland, next month.
"We gave him a very speedy checkup and his weight was one of the pieces of information," Barca doctor Ricard Pruna told a news conference.
"The first impression is that his weight of 64.5 kilograms is on the low side and maybe he is lacking the odd little kilo but I cannot say exactly how much," he added.
"As the season progresses we will get to know the player and we are constantly gathering information. That way we can determine his ideal weight."
