Didier Seeuws was chief-of-staff to Herman Van Rompuy, Tusk's Belgian predecessor as chairman of EU summits, until 2014.
He was a spokesman for Guy Verhofstadt when the liberal leader in the European Parliament, a strong advocate of deeper EU integration, was Belgian prime minister in 1999-2008.
As head of the Special Task Force on the UK, Seeuws moves from running transport, telecoms and energy matters for the EU Council, the body grouping the 28 member states, to coordinate unprecedented divorce talks after Britain's Brexit referendum, Reuters reported.
Detailed legal negotiations with London are expected to be handled by the EU executive, the European Commission headed by President Jean-Claude Juncker. But Tusk, as the chair of the body that groups national leaders, will have to broker the political accords required to drive the process to a final deal.