Brazilian midfielder Casemiro has joined Argentine star Lionel Messi to defend Inter Miami's colors, as the latter announced on Wednesday, but the Major League Soccer (MLS) immediately announced it is investigating allegations that the club violated negotiation rules in this deal.

Casemiro, who played 91 international matches for Brazil and won the UEFA Champions League five times with Real Madrid, became a free agent after Manchester United decided not to renew his contract following a mixed four-year period with the Premier League giant.

The 34-year-old midfielder chose to join Messi and fellow Argentine Rodrigo De Paul at Inter Miami, the club co-owned by Englishman David Beckham.

Inter Miami managing owner Jorge Mas said, 'Casemiro's arrival reflects the vision and ambition that define Inter Miami. We never settle. We always seek to grow, improve, and raise our standards every season.'

He added: 'We have built a global club with a vision to become not only the best club in the United States but also a global benchmark.'

But moments after the announcement, MLS said in a statement: 'We are reviewing allegations regarding a violation of negotiation rules by Inter Miami.'

The investigation concerns 'discovery rights' in the MLS, which clubs must obtain before beginning negotiations to sign certain prominent players.

The LA Galaxy held the discovery rights for Casemiro and were understood to receive compensation from Inter Miami.

The statement said: 'The league is gathering all relevant information and will refrain from further comment until the review is complete.'

It added: 'While Inter Miami and LA Galaxy have reached a settlement regarding the discovery priority for signing Casemiro, its terms will be announced after the conclusion of the investigation into alleged negotiation rule violations.'

While other clubs boast major international stars such as South Korean Son Heung-min (LAFC), signing Casemiro boosts Inter Miami's status as the most star-studded club in MLS.

In December, Inter Miami won its first league title and moved to a brand new stadium in April.

But the club has previously faced issues regarding player transfers.

In 2020, the transfer of Frenchman Blaise Matuidi, a 2018 World Cup winner, from Juventus to Inter Miami prompted an MLS investigation, resulting in a $2 million fine for violating salary cap rules.

The league resumed activity last week after a mid-season break due to the World Cup and hopes to capitalize on the soccer enthusiasm sparked by the global finals, which concluded Sunday with Spain winning the title over Messi and Argentina.

One of the league's promotional campaign slogans after the World Cup read: 'Thank you, world. From here, we'll take it from here.'

Casemiro said in a statement from the club that he is eager 'to win and continue evolving' with Inter Miami, adding: 'The project the club presented to me, along with the effort everyone made to bring me here, means a lot to me.'

Beckham, for his part, said: 'He is a winner who has achieved a lot in the game, and after an amazing career with Real Madrid and Manchester United, I am so happy that he has decided to make Miami his new home.'