The Chevinedge men have a vital fixture at home to table toppers Leigh Miners Rangers (2.30) in their quest to hang on to sixth place and make the play-offs.
But stand off Gudor is not allowed to play because he has been selected to play for Yorkshire against Cumbria on Sunday.
Greenwood, the Fax winger who runs the team alongside Huddersfield Giants' Luke Robinson, said: "It's a ridiculous time to play a county match.
"Lee is a pivotal player for us and one of our most consistent performers this season. Why can't they play the county match in midweek? It's rubbish."
Jason Blackburn has been drafted in to partner Shaun Garrod at half back tomorrow as Siddal look to make amends for a 76-0 defeat away to Miners at the end of September - their worst ever Conference defeat.
Greenwood wasn't at that game but said: "The lads put it down to a bad day at the office. Everything they did ended up with them conceding a try.
"After a defeat like that you either never want to play them again or you want to try to make amends and the lads are really looking forward to this game."
Wigan St Patricks and Oulton Raiders - who meet tomorrow - can both overhaul Siddal if they win their games in hand but Greenwood added: "We have not given up hope of making the play-offs and need to look after ourselves.
"The other teams have to slip up but they have some tough games. However, if we don't win it's probably all immaterial."
Winger Pete Duggan and forward Craig Sanderson return for a Siddal side who drew 12-12 at home to West Hull in their last Conference game a fortnight since.

FIXTURES
National Conference
Premier Division: Siddal v Leigh Miners Rangers.
Division Two: Ovenden v Hull Isberg.

Pennine Supplementary Cup, Semi Finals: West Leeds v Wibsey Warriors, New Earswick All Blacks A v Whitworth Spartans.
Pennine Presidents Cup, Semi Finals: Drighlington v Keighley Albion, Sharlston Rovers v Clayton.
Andrew Bennett Memorial Trophy, Semi Finals: Newsome Panthers v Slaithwaite Saracens, Hoyland Vikings v Lindley Swifts.
Pennine League
Division One: Crigglestone All Blacks v Ackworth Jaguars, West Bowling A v King Cross Park, Siddal A v Moldgreen, Dodworth v Victoria Rangers, Greetland All Rounders v Birkenshaw, Keighley Town v Illingworth.
Division Two: Wyke v Emley Moor, Calder Valley v Thornhill Trojans A, Boothtown Terriers v Ovenden A, Dearne Valley Bulldogs v Stanningley A.
Division Four: Underbank Rangers v Sheffield Hillsborough Hawks, Stainland Stags v Silsden Park Rangers, Hollinwood v Brighouse Rangers A, Queensbury A v Dudley Hill A, Thornton v Meltham All Blacks.
Division Five: Keighley Town A v Wetherby Bulldogs, Halifax Irish A v Siddal B, Keighley Albion A v Drighlington A.
Division Seven: Rochdale Mayfield B v Guiseley Rangers.