18 May, 11:09
IRFU Chief Executive Philip Browne has issued a statement following confirmation of Ronan O'Gara's decision to retire from playing professional rugby.
Ferris is responding well to treatment on his back, while Williams (ribs) will also be given as much time as possible to prove his fitness.
Out-half Niall O'Connor is included in the preliminary squad after his impressive performance for Ulster Ravens in their runaway British & Irish Cup victory over Bridgend.
Roger Wilson played 40 minutes against the Welsh outfit, but he will not be rushed back into action with the senior side.
Robbie Diack and Declan Fitzpatrick have returned to training following their respective injuries, however they will not be considered for selection this weekend.
ULSTER Squad (v Glasgow Warriors) - Backs (12): Paul Marshall, Ruan Pienaar, Niall O'Connor, Paddy Jackson, Luke Marshall, Paddy Wallace, Darren Cave, Craig Gilroy, Andrew Trimble, Tommy Bowe, Jared Payne.
Forwards (16): Rory Best, Rob Herring, Nigel Brady, Callum Black, John Afoa, Tom Court, Adam Macklin, Neil McComb, Dan Tuohy, Lewis Stevenson, Johann Muller, Iain Henderson, Stephen Ferris, Mike McComish, Chris Henry, Nick Williams.