Sigurdsson was Villas-Boas's first signing following his appointment in the summer but the Iceland international has struggled to replicate the form that saw him score seven goals in 18 Premier League appearances at Swansea last year.
"That was excellent for Gylfi," said Villas-Boas. "It has been a hard time for him because at the beginning of the season he was involved a little bit more. But, like Lennon, he is another excellent professional.
"He is working hard to try to get back to a level we recognised last year with Swansea. Against Fulham, we saw him at this level."