Melbourne Cup plans for one-time favourite Puissance de Lune are in doubt after he pulled up lame following his run in the Cox Plate.

The six-year-old entire endured a torrid run in the Moonee Valley classic and his trainer Darren Weir was lost for words after the race.

The big grey shared the running past the post the first time, only to fall back through the field to finish a disappointing 12th.

"He just didn't fire. I don't know why," Weir said.

He also isn't sure whether the horse will run in the Melbourne Cup.

"We'll see how he pulls up, I guess. That's all we can do," Weir said.

"We'll see how the horse is tomorrow."

Long John's trainer Peter Snowden said the Caulfield Guineas winner had his grand final two weeks ago.

"He ran as well as he could. He probably failed to stay on the day, but that's not to say he can't stay later on," Snowden said.

Long John dead-heated for ninth.