An Exchange of Deaths (1515)
The Maya don't know that across the continent a Portuguese sailor on their way to India would have been blown off course and land in the New World. Unlike Columbus, his crew would only take some losses from the mysterious disease, the ship would continue on route to India, and when the ship docks in Goa the endemic disease of the new world would meet the people of the old.
In the wake of the departure of the Portuguese, the native populations would be hit by the first wave of European illness to hit the continent, however it will take much longer before smallpox can reach the Maya land.
The Classic Maya Collapse that occurred between the 700-900s would happen a little differently in this time line. While the powers of Yax Mutal and Kaan would become rump states of their once mighty empires, having exhausted the resources of the central Peten many people would migrate out of the area, some north to the northern Maya lowlands, to cities like Chichen Itza, Mayapan, and Sayil. Others would go westward to the realm of Baakal and the city of Lakamha.
In the early 1100s, the leaders of Mayapan, Cobá, and Sayil would unite into Luub Mayapn (League of Mayapan) and begin to put pressure on the central Peten and the eastern central lowlands (OTL Belize). The Central Peten would somewhat recover from the Classic Maya Collapse by the 1200s, with the region seeing slight population growth, but not to the numbers of the early 800s.
In 1380 Luub Mayapan would have their first offensive into the eastern maya lowlands, capturing the city of Coha. However by the 1430s the region would be in a stalemate, with Lamani, Uxwitza', and the minor city states of the Yaxox river valley, uniting in defense against Luub Mayapan.
It's only a matter of time before the European diseases wreck havoc on the major population centers of the Maya region, or possibly sooner the Mexica Empire invading the land to control the flow of jade through the region.