The Knights playmaker has spent long periods on the sidelines with leg injuries in recent seasons and was hopeful 2017 could be a massive year.

The 29-year-old signed a multi-million dollar deal in 2014 and is contracted until the end of 2018 but could be out for an extended period. 

“It was just a simple drill. I was just squaring up at the line and it just went on me,” Mullen told NewsCorp.

“The frustrating thing for me is that I have been training really well and looking forward to a big season next year.”

Mullen will undergo scans to determine the extent of the damage.