"From what I've read, it doesn't look very likely to me," he told reporters when asked about media reports that a land dispute in the Cancun region could prevent the circuit from being built.
"But I wouldn't be surprised to see a race in Mexico within the life of the current Concorde Agreement, before 2008."
Local organisers announced last October that the Caribbean resort would host a race for five years starting in 2006, marking the country's return to the calendar for the first time since a Grand Prix in Mexico City in 1992.
India
However, the arrival of the country's first Formula One driver, Narain Karthikeyan at Jordan, has revived talk of a future Indian race.
"It depends on the local politicians," said Mosley. "One minute there is great excitement and then they lose the election and it goes away again. I suspect that if Karthikeyan goes well, then you will see one in India."
This year's calendar is already an unprecedented 19 races and Mexico would have been a 20th unless one of the European venues was axed.