The Sharks will host Western Province in the Final in Durban on Saturday, October 28, after an impressive win over the Blue Bulls at the weekend.
Sibusiso Nkosi, who is on the fringe of Springbok selection for the year-end tour, left the field 15 minutes from the end of encounter with the Bulls.
"Looks like he dislocated his elbow," Du Preez told a post-match media briefing.
"He went to hospital, so we will see if there are any fractures."
Prop Thomas du Toit took a knock to the leg, but is not a major concern.
Clarity as to the extent of the injuries are expected on Monday.
Du Preez described his team's 37-27 win over the Blue Bulls in Durban at the weekend as a "fantastic" performance.
"There are a couple of things we know we have to work on, but it is job done," he said about reaching the Final.
"I don't think we have hit our straps yet, so hopefully we will put it together next week.
"We have to sort out or exits and we gave away too many penalties in the second half."
It will be a new experience for a host of Sharks players, with only a handful of them having previously played in a first-class Final.
"We haven't won anything yet," the coach said.
However, he felt captain Ruan Botha has had such a stellar season that he should be up for Springbok selection for the year-end tour.