The 24-year-old Brazilian playmaker left the Ukrainian champions for Anzhi Makhachkala last month.
"Mircea Lucescu is an excellent coach," he told reporters on Tuesday. "He has done a great job at Shakhtar and has taught his side to play attractive football.
"Willian has left Donetsk but that does not mean Shakhtar have become weaker. We have watched the matches they played during their winter break and I can say they have high-quality players. You can hardly point out an area where Shakhtar have a weakness.
"Tomorrow we will see an interesting and intense game because both teams will need to give their best performance or even surpass that level. And, of course, we will do our best - we came here to provide uncomfortable opposition."
Dortmund will head into Wednesday's game on the back of a 4-1 home defeat to Hamburg in the Bundesliga on Saturday.