With a knee injury ruling back rower Stephen Ferris out for four to six weeks, Ulster will start with a new-look back row against Gloucester. Roger Wilson regains the number 8 jersey while Neil Best moves to openside with versatile lock Matt McCullough named as blindside flanker.
Kieron Dawsonâ00s leg injury, sustained in the Glasgow game, is not as serious as initially feared and he is named on the bench as back row cover.
In the battle of the second rows, Ryan Caldwell has been named on the bench with Italian World Cup lock Carlo Del Fava starting alongside former Wallaby Justin Harrison, while the front row of Justin Fitzpatrick, Rory Best and Bryan Young remains unchanged.
McCall has selected an unchanged back line, however due to the injury concern over Humphreys, the Ireland-capped Paddy Wallace is on standby to start at out-half. Humphreys will have a fitness test later today - should he be forced to miss the match, Ulster Academy player Niall O'Connor will come in as the replacement out-half.
Speaking ahead of tomorrow's all-ticket game, coach McCall said: "This game has always been the most important fixture in this part of the season, whether we were coming in to the fixture unbeaten in the Magners League or if in the position we are now in. It is a huge, high pressure game and the players have been looking forward to it for a long time.
"Gloucester got their season off to a flying start playing some great rugby and chalking up some high scores in the Premiership. They have recruited well over the last few seasons and are a strong side and will certainly provide a stern test for us tomorrow night."
A capacity attendance is expected at Ravenhill tomorrow. The Main Grandstand, the Memorial Stand and the Promenade are all sold out, while there are now less than 500 terrace tickets available.
Gates will open at 6.30pm with the kick-off at 8.00pm. Ulster Rugby have brought in a big screen for the fixture which will show Sky footage live from Ravenhill and to add to the atmosphere there will be live music in both the Guinness tent and Scoop Bar as well as children's entertainment including balloon modellers.
Tickets can be purchased online at www.UlsterRugby.com, from a number of Ulster Rugby ticket outlets or from the Ravenhill counter or ticket line on tel: 0044 (0)28 9049 3222 (option 2) between 10am-12 noon, Monday-Friday.
Tickets can also be purchased from various outlets around the province - In Sport - Newtownards; SS Moore's - Belfast; Athletic Stores - Belfast; Eden Park - Belfast; Euro Sport - Lisburn; Shoe Fair Sports - Banbridge; Style N Sport - Ballymena; Vi-Sport - Belfast; David Gotto All Sports - Belfast; Overplay Sports - Carrickfergus; Douglas & Grahame Ltd - Belfast.
ULSTER (v Gloucester): Bryn Cunningham; Tommy Bowe, Andrew Trimble, Rob Dewey, Mark McCrea; David Humphreys/Paddy Wallace, Isaac Boss; Justin Fitzpatrick, Rory Best (capt), Bryan Young, Justin Harrison, Carlo Del Fava, Matt McCullough, Neil Best, Roger Wilson.
Replacements: Nigel Brady, Declan Fitzpatrick, Ryan Caldwell, Kieron Dawson, Kieran Campbell, David Humphreys/Paddy Wallace/Niall Oâ00Connor, Mark Bartholomeusz.