Dufner does not want to see the 25-year-old - who has scored 18 goals in 38 Bundesliga appearances since joining from Manchester United in January 2012 - depart, but is resigned to the fact he may do so.
The Senegal international has been linked with a move back to the Premier League after Stoke chairman Peter Coates said he was one of their close-season transfer targets.
And Dufner admitted that contingency plans are being made at the AWD-Arena for life without Diouf.
"It's quite obvious we are bearing that (new signings) in mind in case Mame leaves. We have a list of 15 or 20 names," the official told Bild.
"I won't comment on names, especially looking out for forwards is not easy. If you want to replace a player like Diouf, you need to have one who is in and scores straight away.
"I would appreciate if he stays with us and extends his contract.
"Let's wait and see how he thinks about it in September."