Petric to miss Fulham first leg
"If all goes well he will be missing for two weeks but the normal recovery period is a full three weeks," coach Bruno Labbadia told reporters on Thursday.
Petric, 29, who was injured in training on Wednesday, has netted five times in the competition this season and scored six goals in the Bundesliga.
Petric scored one of the best goals of the season, volleying home from 15 metres with his back to goal, to help Hamburg win 3-1 at Standard Liege and set up the final four clash against Roy Hodgson's side.
VIDEO: Petric's peach
Petric also headed the first while Peru forward Paolo Guerrero, banned for the rest of the Bundesliga season for throwing a plastic bottle at a fan on Sunday, added the third - his first goal since a cruciate ligament injury in September.