André Villas-Boas has urged the Chelsea board to drive through the purchase of Luka Modric from Tottenham Hotspur as he hopes to bring a fresh dimension to his squad.

The Chelsea manager, who must contend with the loss of the goalkeeper Petr Cech for about a month after he injured his knee in training, has overseen the arrivals of three teenage players so far this summer. The club have finalised the recruitment of Romelu Lukaku from Anderlecht – available for selection for Saturday's Premier League visit of West Bromwich Albion – and he supplements the signings of the holding midfielder Oriol Romeu from Barcelona and the goalkeeper Thibaut Courtois from Genk, who has been loaned for the season to Atlético Madrid, where he will remain despite Cech's injury. Villas-Boas said that he would rely on Henrique Hilário or Ross Turnbull, with the former expected to be selected against West Bromwich.

As well as ongoing interest in Modric, Chelsea are reported to have made a further bid to sign the Valencia winger Juan Mata, with an offer of around £27m set to test the resolve of the Spanish club. An earlier bid of £22m was rejected by Valencia this week but it is believed the player wants to leave. Mata was outstanding for Spain's Under-21 champions this summer and can play on the left or down the middle.

Chelsea are closing in on another teenager, the 19-year-old Genk and Belgium left-winger Kevin De Bruyne. He will cost €10m (£8.75m) but the London club have discussed loaning him straight back to Genk.

The signing of Modric, however, would, according to Villas-Boas, bring a different dynamic. Villas-Boas did not want to speak expansively on the subject but he did confirm that he had held discussions with the Chelsea chief executive, Ron Gourlay, and the technical director, Michael Emenalo, on how best to proceed. He said that he was in charge of "technical" decisions in relation to signings and it was clear that he wanted the club to do all they could to secure Modric, although he did concede that they should not be made to pay a fee that was not sensible.

The Tottenham chairman, Daniel Levy, has been categorical that he will not sell Modric but the player is keen to complete the transfer and Harry Redknapp said on Wednesday that, if the club did cash in, he would be able to add "four players and have a better team".

Chelsea have had bids of £22m and £27m rejected for Modric but they are preparing an increased offer, which Villas-Boas hopes will close the deal. "It depends on the club's decision, if they are willing to go forward or not, up to which value and whether they want to keep the interest in the player," he said. "It doesn't depend on me. It's not my money. We have to take sensible decisions or else you end up criticising us for other stuff. [But] the technical decisions are mine, of course.

"It depends on what you have available. On the price … on the age of the player that you get. Luka, on this particular occasion, is a young player but there would be, for sure, a sensible price to pay."

Villas-Boas, who also has an interest in Valencia's Juan Mata and Liverpool's Raul Meireles, was asked whether signing an established star like Modric, rather than teenagers with potential, would bring a different dynamic to the group. "Hopefully," he replied, before he added: "I think the ones we have are superstars.

"With Michael Essien's injury for six months, it's fair to say that we might go to the market regarding a midfielder. For the first competitive games and bearing in mind the group phase of the Champions League and bearing in mind the start of the Carling Cup, we might just add a number in midfield. We have been studying all kinds of options.

"We have approached Tottenham before regarding the player [Modric]. I don't know if it's viable or not and to what extent we can go regarding his price because we are speaking about a high amount and nothing else basically."Cech fell awkwardly on his right leg after an aerial challenge in training and Villas-Boas admitted that he feared the worst, describing the incident as "pretty nasty". An MRI scan, though, has revealed that the medial ligament in Cech's knee is stretched rather than ruptured. "The good news is that it's just three to four weeks," Villas-Boas said.

Villas-Boas reported that Mikel John Obi had made himself available for the West Bromwich game, despite the ongoing trauma of his father being held by kidnappers, who are demanding an £80,000 ransom in his native Nigeria. The manager also said that David Luiz would return to full training next week, after thigh trouble, and Didier Drogba was certain to agree his one-year contract extension.

"He is one of the most important and representative players at the club and his willingness to continue is 100%," Villas-Boas said. "It will be because of this that we will reach an agreement."