The Bundesliga set August 28 as the date to resolve Havertz’s future, before the start of preseason.
Lampard’s first-team squad are scheduled to return next week and his coaching staff left for their holidays believing the player would have signed for the club by the time preparations for the 2020/2021 campaign began.
Negotiations are ongoing between the two clubs, with Leverkusen finally showing signs of budging from their £90m valuation.
Chelsea have submitted two offers and although they hoped to land the 21-year-old for £70-75m, the total figure will now be slightly higher as they edge closer to a compromise.
Lampard is not involved in negotiations, with director Marina Granovskaia and technical advisor Petr Cech leading the chase, but is keen to develop an understanding between Havertz, Timo Werner and Hakim Ziyech ahead of the new campaign.