The 19-year-old, who is on loan from Juventus, sustained the horrific injury in the 16th minute when he slid in to challenge for the ball with Radja Nainggolan.
Mattiello planted his foot on the ball, before his leg bent to almost a right angle upon contact with Nainggolan's outstretched limb.
The Roma man escaped punishment - and replays indicate that decision was correct - while Mattiello was replaced by Valter Birsa.
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'Andy Robertson has a decision to make as a player, where you either roll your sleeves up and give someone great competition or you accept decisions and say that your time is done.' Alan Shearer on the conundrum facing Arne Slot's Liverpool
