Brentford striker Scott Hogan has suffered a setback in his recovery from a long-term knee injury and will require further surgery.
The 23-year-old suffered cruciate ligament damage in August and returned to light training earlier this year.
Hogan injured the same knee in training on Tuesday, and the Bees say the damage is "of a significant nature".
"At this stage the timescales for recovery are difficult to predict," Bees head of medical Neil Greig said.
"I would anticipate knowing more on that front following surgery, which is likely to take place next week.
"We will now reassess our aims and refocus on setting new goals for Scott's immediate and long-term care."
Hogan joined Brentford from Rochdale for an undisclosed fee last summer and suffered the original injury, in his left knee, in only his second appearance for the Championship club.
He had been participating in low-level team training for the past month, with a view to being fully-fit for pre-season training.
"It happened during an innocuous passing drill and there was no-one near him," Brentford manager Mark Warburton told the club website.
"This is a devastating blow for a young guy that has worked so hard over the last eight months, along with the medical team, to get back to full fitness."