Hawthorn spearhead Mitch Lewis will miss the start of the AFL season because of a knee injury.

Lewis, who emerged as one of the AFL's best key forwards in a breakout season last year, has suffered a sprain to an anterior cruciate ligament in a knee at training.

An early prognosis suggests the 24-year-old will not require surgery but a timeline on his return is unknown, Hawthorn's head doctor Liam West says.

"He will certainly spend some time on the sidelines though and, at this stage, we expect him to miss round one," West said in a statement on Thursday.

Lewis booted 37 goals from 15 games last year but was sidelined for the last three games because of jarring to the same knee he has again injured.