Ulster have suffered a significant injury blow just one week into the season, with John Cooney sustaining a hamstring injury.
The scrum-half lasted just 30-minutes of the 35-29 win over Glasgow Warriors on Friday night, which opened their United Rugby Championship campaign.
Cooney was replaced on the night by Nathan Doak.
Ulster confirmed on Monday that the Ireland scrum-half suffered a hamstring injury at Kingspan Stadium, and requires an MRI.
19-year old Doak scored Ulster's fourth try and kicked a trio of conversions after coming on for Cooney.
Meanwhile, winger Robert Baloucoune will miss Saturday's trip to Zebre after suffering a tightness to the groin in the wake of the Glasgow win.
Cooney and Baloucoune join the already injured Iain Henderson (thumb), Kieran Treadwell (shoulder), Cormac Izuchukwu (knee), Luke Marshall (knee), Jack McGrath (hip), Jordi Murphy and Ian Madigan.
Head coach Dan McFarland was pleased with his side's "outstanding" effort on Friday, but was perplexed by a yellow card shown to James Hume which resulted in a penalty try for the Warriors.
Hume's try-preventing challenge on Rufus McLean led to a lengthy conversation between referee Ben Whitehouse. The officials deemed the centre had dived onto McLean, leading to a yellow.
"I would wonder what he's meant to do?" McFarland posited afterwards, "I don't think James played the man on the floor, I think he played the ball, he went to try and hold the ball-up.
"What's he meant to do? In the history of rugby, I don't think I've ever seen that penalised in the act of scoring.
"I've been in rugby a long time, and I don't think I've ever seen that before. I could be wrong.
"Maybe it's to the letter of the law but we all know the game isn't reffed to the letter of the law.
"I'm not cross about it because we won but it's a really interesting situation."