The Werder Bremen loanee joined the donor register seven years ago and will now give blood to generate stem cells for a transplant.
If successful, the treatment could save the life of a patient in need.
"The chance of a full DNA match is remarkably small, which is the reason why the situation is so very special," VVV Venlo said in an official statement.
"Lennart will be absent in Venlo for the whole week and pick up his training after the weekend."
The 26-year-old German striker has scored seven goals in 27 appearances for Venlo this season, but it's off the pitch where he may make his biggest impact.
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