Moutinho had an impressive Euro 2012 campaign with Portugal in June and helped his country reach the semi-finals of the competition.
The 25-year-old is rated as one of the more dynamic midfielders in European football, and fellow Portuguese Villas-Boas has conceded his admiration of the former Sporting Lisbon star.
"It will be very difficult to sign him," he said.
"I have experience of Porto and it is a club that sells good products but they are expensive.
"It has been like that with numerous players and that is why it is very difficult."
Tottenham could lose playmaker Luka Modric this summer and Villas-Boas has already eyed potential replacements for the Croatian.
"The targets are identified, it's just a question of how quickly we can close the deals," he revealed.
"It will be dramatic for the club if we cannot find the right replacement but we are looking to strengthen the squad immediately if eventually the transfer goes through."
Tottenham will begin their 2012-13 Premier League campaign with an away game against Newcastle United on Saturday.