There are 10 changes in total to the starting lineup that ran out in Dublin last weekend, as Franco Smith's squad travel to Italy knowing that a victory would strengthen their pursuit of home advantage in the playoffs, as well as securing the Scottish-Italian Shield for a third consecutive season.
An all-new front-row sees Hiddleston start at hooker, the Dumfries Saints product packing down between Jamie Bhatti and Fin Richardson at the coal-face.
Alex Samuel retains his place in the engine room from last weekend's defeat in Dublin, with Jare Oguntibeju coming in for his first appearance since last month's victory over Munster at Scotstoun.
Euan Ferrie and Jack Mann come into the back-row, as Sione Vailanu moves to openside to accommodate the inclusion of Mann at number eight.
Behind the scrum, Jamie Dobie starts at scrum-half after three successive starts on the wing, partnering Adam Hastings in the half-backs for the trip to Parma.
Stafford McDowall shifts from outside to inside centre, with Ollie Smith coming into the midfield to start in the number 13 jersey.
Kyle Steyn captains the squad from the wing once more, in a back-three rounded out by the returning McKay and Argentinian flyer Sebastian Cancelliere.
The BKT URC's top try-scorer forms part of a five-three split of forwards and backs amongst the replacements, as Johnny Matthews joins Patrick Schickerling and Sam Talakai as the front-row cover.
Max Williamson retains his place on the bench after returning from injury last weekend, with Rory Darge rotating to the bench to cover the back-row.
Tom Jordan is another to rotate to the replacements after starting last weekend, whilst Sean Kennedy and Facundo Cordero join the Scotland international to round out the matchday 23.
Speaking following selection, Head Coach Franco Smith told glasgowwarriors.org: "Zebre are a team that can hurt any team when presented with an opportunity, and we have seen the significant progress they have made across this season both in Parma and on the road.
"We are pleased to welcome Josh and Gregor back into our matchday squad, with both men having worked diligently with our medical and S&C teams to put themselves in the best shape possible.
"We know that there is little margin for error in a league as tightly contested as the URC, and we know we must be at our best tomorrow night."
15 Josh McKay (61)
14 Sebastian Cancelliere (55)
13 Ollie Smith (42)
12 Stafford McDowall (88)
11 Kyle Steyn (Captain) (86)
10 Adam Hastings (64)
9 Jamie Dobie (83)
1 Jamie Bhatti (119)
2 Gregor Hiddleston (21)
3 Fin Richardson (4)
4 Jare Oguntibeju (7)
5 Alex Samuel (34)
6 Euan Ferrie (33)
7 Sione Vailanu (32)
8 Jack Mann (11)
Substitutes
16 Johnny Matthews (92)
17 Patrick Schickerling (9)
18 Sam Talakai (17)
19 Max Williamson (26)
20 Rory Darge (61)
21 Sean Kennedy (31)
22 Tom Jordan (65)
23 Facundo Cordero (9)
Unavailable for selection: Ben Afshar (foot), Scott Cummings (arm), Jack Dempsey (hamstring), JP du Preez (head), Matt Fagerson (ankle), Zander Fagerson (calf), Angus Fraser (ankle), Huw Jones (ankle), Ally Miller (back), Grant Stewart (illness), Rory Sutherland (shoulder), Sione Tuipulotu (pectoral), Henco Venter (suspension), Duncan Weir (shoulder).
Referee: Peter Martin (IRFU)
AR 1: Federico Vedovelli (FIR) AR 2: Riccardo Angelucci (FIR)
TMO: Mark Patton (IRFU)