The second round, second leg, played at the neutral venue of Salerno, was called off at half- time after a linesman, Philippe Lament, was injured by a firecracker thrown on to the pitch. Fiorentina were leading 2-1 on the night and 4-1 on aggregate at the time of the incident.
Uefa, European football's governing body, has postponed a decision on the future of the tie until a meeting on 9 November.
Previous precedents set by Uefa suggest that Fiorentina risk, at worst, elimination from the competition, with a 3-0 technical victory awarded to Grasshopper or, at best, being allowed to replay the tie behind closed doors.
The match was being played in Salerno because Fiorentina are still serving a two-match ban from their own ground in Florence after violent incidents at a Cup-Winners' Cup semi-final with Barcelona in April 1997.
Fiorentina officials suspect Tuesday's incident may have been provoked by rival Salernitana fans, keen to exact revenge after violent scuffles in Fiorentina's 4-0 Serie A home win over their team in October.
Last week Salerno police warned that the Uefa tie was a high-risk match and called for it to be played at another venue. But the game went ahead in Salerno partly because it was too late to change the venue and partly because Fiorentina had decided to donate the gate takings to victims of the Sarno mudslides of last May.Reuse content