Reyna assists three in BVB rout without Sancho
Reyna, the 17-year-old American, served up three assists in the absence of their top provider Jadon Sancho, sidelined with an infection.
Norwegian forward Haaland scored two of those goals either side of Emre Can‘s 47th minute header, to take his tally to 17 goals from 18 Bundesliga games this year.
Haaland then set up Felix Passlack for their fourth in stoppage time.
It was the first win in three games for the Ruhr valley club after last week’s shock loss to Augsburg and Wednesday’s defeat to Bayern Munich in the German Super Cup.
Dortmund have six points from their three league games, with Eintracht Frankfurt top on seven after their 2-1 win over Hoffenheim.