The 18-year-old has had a breakthrough season in Bremen, starting 20 of their 28 Bundesliga games and establishing himself as one of the most sought-after young defenders in Europe.
Sky Germany report Chelsea are the most advanced of the interested clubs, having already held discussions with Coulibaly's entourage. Fabrizio Romano describes a "big race" underway, with scouts from multiple top sides now regular fixtures at the Weserstadion.
Werder are not looking to be pressured into a low-ball deal.
Managing director Clemens Fritz is holding out for between €40 and €50 million — a fee that would represent a record sale for the north German club. Coulibaly is contracted until 2029, which gives Werder considerable leverage, and Fritz has made clear he will demand a "mega sum" before agreeing to any deal.
The interest is easy to understand.
Coulibaly is 6ft 3in, left-footed and fast — a rare combination of physical dominance and technical quality. He is comfortable in possession, reads danger well and can drop into midfield when required. Born in Oldenburg of Ivorian descent, he came through Hamburg's academy before joining Werder in 2024 and has already earned caps for the Germany Under-21 side.
He scored on just his second senior appearance, netting in a 3-3 draw against Bayer Leverkusen last August, and hasn't looked back.
For Chelsea, the need is obvious. Levi Colwill has been missing since a pre-season ACL injury and the club failed to land Jeremy Jacquet in January, with the Frenchman choosing Liverpool instead. Liam Rosenior's side have conceded costly defensive errors all season.
Coulibaly, still only 18, would not be an immediate fix. But on Chelsea's model — long contracts, raw talent, patience — he fits precisely.