The Belgium international came on as a half-time substitute during Tottenham's 3-1 pre-season defeat to Sunderland in the Asia Trophy at the Hong Kong Stadium on Wednesday.
But he lasted just six minutes before slipping and injuring his left ankle on a pitch badly-affected by terrential rain.
And Villas-Boas is fearful the 26-year-old may have sustained ligament damage in the incident.
"He might have sprained the ligaments in his left ankle," he said.
"He will need to have an MRI scan to establish the full extent of the problem.
"But there is some range of movement, so we have to hope it won't be that long. Hopefully he'll be back for the first game of the season.
"I was concerned (about the conditions) but what can you do? It isn't that great, the pitch is what it is.
"There are often problems here, but you can't change the conditions. They are extremely poor."
