NHL superstar P.K. Subban was slashed with a skate so badly during a game Sunday ... the dude suffered a massive thigh wound -- and the pics of his gross injury are GNARLY!!!

The 31-year-old New Jersey defenseman shared the images of his leg after he was cut during the Devils' game against the Penguins ... and you can see, the wound is deeeeep.

In fact, it was so bad ... P.K. had to get stitched up mid-game!!!

Don't worry ... Subban's fine, he even wrote of it all that he didn't lose any time on the ice despite the cut, explaining, "Never missed a shift..."

As for how it went down, video of the accidental slash in the Devils' 5-2 loss to Pittsburgh hasn't been unearthed yet, but Subban explained it this way on his social media ... "Skate blizzy caught me slippin."

Guess that's one way to put it.