It is definitely Groot. It’s a regular occurrence in the Guardians of the Galaxy comics for Groot to be blown up and only a splinter of his remains are required to regrow him. It was even used as a plotline from the recent Secret Wars storyline where Peter Quill (from the main reality) kept a splinter that he carried around in his mouth like some people do with a toothpick. Here’s him using it here- by sticking it into Dr Doom’s “throne” which is actually made from Yggdrasill, the Asgardian World Tree.
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