Ryan Reynolds is dishing on why Rob McElhenney doesn't appear in the new 'Deadpool' flick ... saying they couldn't make his scene work -- which wasn't Rob's fault at all.

The movie star posted about his close friend and fellow Wrexham A.F.C. co-owner Friday on Threads ... acknowledging his buddy did film a scene for the flick -- but, they couldn't figure out how to fit it in the final cut.

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The way RR explains it, the sequence itself didn't work ... and, the only reason it didn't immeditaly end up on the cutting room floor was because of Rob's performance.

Reynolds says he brought a great intensity to it, and he appreicates his close friend flying all the way to London for him ... but, it just wasn't meant to be.

Ryan did reveal the fate of Rob's character though ... admitting a huge creature ate him and eventually excreted him -- not exactly a heroic end for Rob.

We already knew Rob's cameo ended up getting cut out -- 'cause he let everyone know pretty much the day the movie came out.

He shared a pic posing alongside Hugh Jackman on set, joking that the theater he went to must've accidentally cut his part out, since he flew 6,000 miles to film it, and he just knows Ryan wouldn't do that to him.

FWIW ... "Deadpool & Wolverine" is back on top of the box office in its fifth weekend and Wrexham has won 2 of its first 3 league matches -- so, we bet the two pals are pretty stoked, even with the unfortuante editing.