The 22-year-old will have an operation in two weeks, which is expected to sideline him for nine months.
Brentford have not disclosed the full details of the injury, which was sustained during their victory against QPR on Friday.
Macleod was stretchered off in the 87th minute of Brentford's 2-0 win at Loftus Road, having come on as a substitute.
The former Scotland Under-21 international was previously ruled out for a year because of various injuries after he joined the Championship club from Rangers in January 2015.
Brentford midfielder Lewis Macleod will require reconstructive knee surgery and has been ruled out for the season.