Ferris, 26, hobbled off after 33 minutes of Ulster's 45-7 RaboDirect PRO12 win over Aironi at Ravenhill and is having a scan to assess the damage today.
Despite only playing four PRO12 games for his province this season, Ferris is one of McLaughlin's star players and will be crucial to Ulster's chances at Thomond Park.
And the Ulster supremo is hoping that the results of the scan will not be as bad as initially feared.
"Stephen rolled an ankle and it's swollen. It's probably worse than I thought," said McLaughlin.
"Obviously as the week progresses we'll know more, but yes he's going to be a bit of a doubt.
"We've got an extra couple of days as it is a Sunday game, which could stand us in good stead. But we're worried, and that's quite simply the way it is.
"When you're playing against the Italians it's always extremely physical.
"There are always going to be bumps and bangs, but everybody else we brought off, we brought off as precautions and to rest. Stevie, at the minute, would be our major concern."
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