On-loan Tottenham midfielder Bentley has been troubled in recent weeks by a knee problem sustained during the FA Cup quarter-final draw at Millwall, since when right-back Orr has not played due to a rib injury.
Both men have stepped up their recoveries over recent weeks, but the game comes too soon for Martin Olsson (ankle).
Grant Hanley and Colin Kazim-Richards complete two-match suspensions having collected 10 yellow cards and Gael Givet is set to continue at centre-back after impressing in place of the former during Tuesday night's 1-0 win over Huddersfield.
Goalkeepers Jake Kean (knee) and Paul Robinson (back/lung) head up a list of long-term absentees that includes Adam Henley (hamstring), Dickson Etuhu (knee) and DJ Campbell (foot).
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- Martin Olsson