His talents were on show again in
Wednesday night's 2-1 win against Liverpool
as he set up fellow wide-man Aaron Lennon for the opener following a pacy burst down the left, before deceiving goalkeeper Pepe Reina with a swerving free-kick to get on the scoresheet himself.
Modric said Bale should join Real to fulfil his Champions League aspirations but Villas-Boas insisted: "The club has to protect our assets.
"We have to qualify for the Champions League, as Luka says, and for that to happen, we need the players that can make a difference.
"In the end we are not looking to sell. We are first looking to our objectives towards this season and that is the most important thing."
Villas-Boas also highlighted the effect that increased competition for places at other clubs can have on playing opportunities.
Modric, who was guaranteed a starting spot while with Spurs, has seen himself rotated with Germany international Mesut Ozil in the playmaker role at the Bernabeu since joining for approximately £30m in August.
He said: "Regarding Luka, he has high expectations and I think Real Madrid have high expectations for him too. I think with the player that he is, he will turn it around and will manage to get into the team a little bit more."
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