The newborn son of FC Barcelona star Lionel Messi is already registered as a member of the Argentine club his dad supported while growing up in Rosario, fans of Newell's Old Boys proclaimed at a match.
"Welcome to the world, Thiago Messi ... to the World of Lepers. Member No.2,881,152," said a banner held aloft by Newell's fans during Sunday's 1-1 tie with Godoy Cruz in the western city of Mendoza.
Newell's fans are traditionally known as lepers, while supporters of crosstown rival Rosario Central proudly call themselves rabble.
The nicknames are said to have originated a century ago, when the two clubs agreed to play a benefit match for a hospital treating lepers, but only Newell's actually turned up for the contest.
Leo Messi cheered for Newell's as a boy and played on the club's youth team before signing with Barcelona at the age of 13.
Thiago Messi Rocuzzo, the first child of Leo Messi and Antonella Rocuzzo, was born on Friday at Barcelona's USP hospital.
"Today I'm the happiest man in the world, my son was born and thank God for this gift," the 25-year-old striker wrote on his Facebook page.
Messi recently received the Golden Boot as the top scorer in European national leagues, thanks to his 50 goals last season with Barcelona in La Liga, and was named as one of the 23 athletes in contention for the Ballon d'Or, FIFA's award for World Player of the Year.