The league has let the requirement slide in the past. New York City F.C. was announced as the league’s 20th team last year, after paying a reported fee of close to $100 million, but it still does not even have a temporary home for its first season, in 2015. Orlando City, which was named M.L.S.’s 21st team in November, has a stadium deal in place and will also begin play in 2015.

M.L.S. is sure to welcome Beckham as an owner, but getting a stadium for the team in Miami will not be easy, because of political pressures and a vibrant real estate market that has driven up prices.

Discussions have centered on a piece of county-owned land near the city’s seaport, PortMiami, where cruise lines and cargo ships dock. But that property was slated for commercial development, and putting a stadium there instead could represent a loss of revenue for the county. To win county approval, the owners of the stadium might have to make market-rate payments, said one person with knowledge of the thinking of county officials.

Even then, it is unclear how soon a stadium might be built, though the team could presumably play a year or two at a temporary home like the Miami Dolphins’ stadium in Miami Gardens or at Florida International University in Miami. New York City F.C. has said that it plans to take that approach and that it expects to announce a temporary home in the next few weeks.

There are also political hurdles for Beckham’s Miami team. Carlos A. Gimenez, the mayor of Miami-Dade County, has opposed the use of public money for privately operated sports facilities. Before becoming the mayor, he was one of the few county commissioners to oppose the use of hundreds of millions of dollars in public money to help the Miami Marlins build a stadium, and he questioned whether the Dolphins should be given tax dollars to refurbish their privately owned stadium. Reaction to the deal to help the Marlins cost several top politicians their jobs, and Gimenez, who will attend Wednesday’s news conference, would be loath to change course suddenly to help build a soccer stadium.