Chelsea will have to exclude one of their big-name signings this January from their Champions League squad.
The Blues will play Borussia Dortmund in the last-16, with the first leg in Germany on February 15 and the return leg at home on March 7.
Owner Todd Boehly completed a splurge this month with Enzo Fernandez becoming their eighth signing in a British record £107 million deal on deadline day.
Other arrivals include Joao Felix on loan from Atletico Madrid, £88m Mykhailo Mudryk and £35m Benoit Badiashile.
However, UEFA rules stipulate only three new players can be registered for the knockout stages.
It means Chelsea must decide which three out of the eight new faces Graham Potter will add to their squad before Thursday night.