The original plan was for Van Rompuy to visit Slovenia as Belgian prime minister. After he was selected president of the European Council, Yves Leterme took over as Belgian prime minister on Wednesday.
Van Rompuy will officially take over as EU president on 1 January, which means he will pay the half-day visit to Slovenia as the president-elect of the European Council.
Van Rompuy is already getting ready to assume his new tasks and his talks with Pahor are expected to revolve around the European Council's future work.
The pair might also touch on the issues on the agenda of the December EU summit, which is to focus on climate change, financial oversight and the Stockholm programme, which is to define the framework for EU police and customs cooperation.
The European Council is also to debate the Western Balkans and EU enlargement, which are both Slovenia's priorities.
The position of the European Council full-time president is introduced by the Lisbon Treaty, which steps into force tomorrow. The treaty also turns the European Council into one of the EU institutions.
Already today, Van Rompuy is visiting Sweden, while tomorrow he will continue his tour by meeting Italian Prime Minister Silvio Berlusconi in Milan. On Tuesday evening, he is also to attend the ceremony marking the Lisbon Treaty's entry into force in Portugal.
The 62-year-old Van Rompuy served as Belgium's prime minister for less than a year, in which time he was credited for reducing tensions between the Flemish and Walloon communities that threatened to split the country.
Pahor said upon his appointment that it is important for Slovenia that the new president of the European Council will be a leader of a relatively small country. He also described Van Rompuy as a "man of compromise, a man of dialogue - this is obviously what Europe needs right now".
The Slovenian prime minister however rued a missed opportunity by the EU to appoint a "strong European leader". "The time has not yet come for strong European leaders," Pahor said after EU heads of state and government hammered out a consensus deal on 19 November that made the globally little-known Van Rompuy the new face of the bloc.