Former Porto boss Villas-Boas is keen to be reunited with Pereira after their successful time together in Portugal last season, but the two clubs remain some way apart in their transfer negotiations.
"The only thing I can confirm to you is that we have made a bid for him and we are very far apart in terms of numbers," Villas-Boas said.
When asked if he was confident a deal could be done, he added: "Not very confident."
Pereira, 25, is regarded as an emerging talent after helping Uruguay win the Copa America and also playing a part in his country's run to the 2010 World Cup semi-finals.
If Chelsea were able to conclude a deal, Pereira would compete with Ashley Cole for the left-back berth and could also fill in at right-back.