Demeble missed his side's 4-2 home defeat to Chelsea on Saturday after a persistent hip problem flared up on international duty.
Villas-Boas revealed that the Belgian playmaker has a problem that is proving problematic to diagnose and treat.
He told the Daily Mail: "He had a scan, but we haven't seen the results yet. He suffers discomfort and pain on his hip which is not something that shows on the scan so it's difficult to assess.
"It is something that comes back to him. He fell awkwardly in the Scotland game against Belgium and felt the pain.
"He had a chance to play (on Saturday) and had a late fitness test but the pain didn't go away."
The former Fulham midfielder is unlikely to travel to Slovenia with the Tottenham squad this week as they continue their Europa League campaign with a trip to Maribor.