The Stamford Bridge side were ready to negotiate late last month after their qualification for next season's Champions League was confirmed.
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Romelu Lukaku scored the winner in a 2-1 victory over Bayer Leverkusen to lead Inter Milan nto the Europa League semifinals on Monday.
Chelsea are ready to back head coach Frank Lampard in the transfer market this summer and have identified Kai Havertz as their next target.
The Blues are thought to be confident of securing a deal in the region of £70-£75 million despite Leverkusen demanding closer to £90m.
The Europa League draw made on Friday means Manchester United face a potential final with Inter Milan.
Chelsea are yet to agree a fee for the 21-year-old but remain hopeful they can drive down the £90 million asking price.
Bundesliga champions Bayern Munich secured their second consecutive domestic double on Sunday with a 4-2 win against Leverkusen in the German Cup.
Bayern Munich can seal a league and cup double on Saturday, but Leverkusen have beaten them once already this season. Can lightning strike twice?
Chelsea head coach Frank Lampard has said he admires Kai Havertz but the club are yet to table any offers for the Bayer Leverkusen forward.
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Leon Bailey continues to impress in Germany and Manchester City, Chelsea and Man United have taken notice. Transfer Talk is LIVE with the latest.
Chelsea have been busy in the transfer market already and Bayer Leverkusen's Kai Havertz could be next. Transfer Talk has the latest.
Bayer Leverkusen came from a goal down to snatch a 1-1 draw at Schalke 04 on Sunday as they held on to fourth place in the Bundesliga.
Bayer Leverkusen beat Saarbrucken 3-0 away on Tuesday to reach next month's DFB Pokal final and end the fairytale run of the fourth-division side.
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