The Chilean believes that the Champions League quarter-final tie is "absolutely open" heading into next week's second leg at the Westfalenstadion.
However, if Los Boquerones are to progress, they will have to do it without skipper Weligton and midfielder Manuel Iturra.
"Dortmund are an excellent side," Pellegrini told UEFA.com. "It was an open game and the tie is absolutely open.
"Dortmund's team are all good young players with great technique and dynamism.
"[Weligton and Manuel Iturra] are important players; luckily [Martin] Demichelis didn't get a yellow card as well, so we will see how we replace those two."
The Andalusian outfit are in La Liga action at Real Sociedad on Saturday before the return leg on Tuesday.