Accurate projection of future meteorological drought requires understanding of contributions from different climate drivers (e.g., CO2, other greenhouse gases, land use and land cover change, and aerosols). For the Amazon, much of the recent analysis on drought has focused on climate change caused by increasing atmospheric CO2 and other greenhouse gases. The role of land-atmosphere coupling still remains uncertain. In this study, we leverage model simulations of idealized deforestation and increasing CO2 from the 6th phase of the Coupled Model Intercomparison Project (CMIP6) to quantify the transient response in Amazonian precipitation and air temperature during 21st century to deforestation, physiological responses to increasing CO2, and radiative effects of increasing CO2. We find that a 10% loss of forest cover is comparable to the physiological effects of a 100 ppm CO2 increment with respect to impacts on basin-wide mean rainfall. Together, deforestation and CO2 physiology together account for 41-49% of future decline in Amazonian rainfall by 2100 in CMIP6 model projections for widely used Shared Socioeconomic Pathway (SSP) scenarios. In contrast, deforestation and CO2 physiology have only a minor effect on levels of future warming. Our results highlight the important role of land-atmosphere coupling in contributing to future meteorological drought in the Amazon and the potential for misattribution of drought effects in studies that use simulations from CMIP6 SSP scenarios, but assume that the climate forcing arises primarily from greenhouse gas and aerosols.