Raper, 48, who was in charge of Castleford from 1997-2001, offered to return from Australia to help out while the club continue their search for a successor to Ian Millward, who left the club earlier this month following a run of poor results which left the Tigers bottom of Super League.
Tigers chief executive Steve Gill said: "We thought long and hard about Stuart Raper's offer to work with Danny Orr until the end of the season, however, we have decided not to go down that route. I think the financial outlay that would have been required to bring Stuart over could be used more wisely on a long-term appointment."
Gill has expressed his desire to appoint a British coach and former Castleford players John Kear, now in charge of Batley, and Andy Hay have been linked with the vacancy, along with Featherstone coach Daryl Powell and Leeds assistant James Lowes.
Hull have declined to comment on a report earlier this week that Hay had resigned his position as assistant to Black and Whites head coach Peter Gentle.
Gill added: "We're progressing with our search for a new head coach. We have a total of 10 applicants to date and we are busy assessing and meeting with them.
"We will make a further announcement in due course."
In the meantime, Orr is preparing the side, with help from youth development chief Richard Tunningley, for Saturday's game against Salford.
- Sports & Recreation
- Stuart Raper