The NSW Blues may have suffered a major crisis before Game 2 of State of Origin, with star halfback Nathan Cleary suffering a hamstring injury on Sunday evening playing for the Penrith Panthers.

In his first game back since Wednesday night, Nathan Cleary left the field within the first 20 minutes and headed to the dressing rooms.

It was then confirmed he endured a hamstring injury ruling him out of the remainder of the clash at home against the St George Illawarra Dragons; however, the severity is not yet known. The halfback is set to undergo scans tomorrow, with early reports that the club are hopeful of a low-grade injury.

"Just spoke to the Panthers medical staff. Obviously, Nathan Cleary (is) not coming back tonight, (and) is a hamstring. (He) will get scans tomorrow. They are hopeful it's a low-grade hamstring injury," Jake Duke said on the sideline for Fox Sports.

This was followed by the Fox SportsĀ commentary team announcing he is a "1000-to-1 chance to play Origin."

If anything is to go by, the body language shown by Cleary on the field was not positive, and he knew he had tweaked something. This was followed by not returning back to the field and changing out of his Penrith jersey.

The Panthers trailed 8 points to 6 after half an hour.