Long-time reader of stories, first time writer.
Criticism is welcome.

When Izuku awoke on a thin, paper-covered, mattress, with a needle in the back of his hand, a cuff on his wrist, and a freight train running through his head, his first thoughts were not the 'Where am I, what happened, why does my mouth taste how a sewer smells?' gamut which would be expected of him. His actual first thoughts went something like 'I HAVE LEGS!'

To understand why he thought this we must travel 3 hours previous and 6 blocks southwest to the grounds of Orudera middle school where Izuku finds himself on the ground, facing the sky, in a daze within reaching distance of a small koi pond.

Rising, unsteadily, to his feet, using the aforementioned concrete structure, he glances briefly at its contents and reaches towards his Hero Analysis/clearly not fish-food notebook. Shooing a few curious koi from nibbling at the waterlogged, yet singed, cover, he clutches his hard work to his chest, thanking his lucky stars that his stationery choices have tended to be weather resistant after past mishaps.

Ignoring his surroundings (an effort he would later find himself quite impressed by) he began to journey in the vague direction of home. Coming to the underpass he makes use of, due to cutting down his travel time to and from school, he realises, with a start, that he is not in possession of his backpack. Wondering how he could have made it so far without realising he was unencumbered by the mass of several textbooks, he failed to notice the dull scraping sound of a manhole cover shifting.

There are 204 bones in the body of an average, adult, human (without a mutant type quirk) and roughly 300 at birth. When the slime villain forced itself around Izuku's face and torso, it broke 2 of them, with the force of so much fluid impacting such a small area.

The shock and fear Izuku felt when the villain appeared was quickly dwarfed when the excruciating pain from his sternum and part of his clavicle snapping, momentarily ceased only to be replaced by a deadened feeling of his chest cavity stretching. He would have attributed this to the criminal currently attempting to invade his nose and mouth but the villain's yell of "What's the big idea", coming through the viscous fluid in his ears said otherwise and added confusion to his list of emotions overwhelming his conscious thoughts.

Having 96% of the bones in your body suddenly liquefy and all of your muscles and cartilage suddenly expanding and reshaping into something unrecognisable is an experience few can admit to having experienced, fewer would be able to articulate and fewer still would even want to attempt to do so. With his nervous system stretching and rearranging to an utterly alien configuration Izuku could only flail and try to scream as the enormous mass of flesh and sinew that made up his being filled the limited space provided by the underpass before sealing off one entrance. Feeling a wriggling near the nerve clusters he used to attribute to his left arm, he had a sudden realisation that the sludge villain was trapped between the concrete wall and the, now oversized, wall of skin that he thought was his front.

Unable to see due to not knowing the location of his eyes (if he still had any), he was unable to see the top hero of Japan exit the same manhole the villain had but was certainly still able to hear the startled, English, exclamation of "HAVE NO FEAR FOR- WHAT THE FUCK IS THIS‽"

In any other situation Izuku might be impressed that All Might could pronounce an interrobang and would be somewhat proud of himself for knowing what that was but at this moment he could only focus on figuring out if he had vocal cords and if he could use them to ask all might for help.

Unfortunately for the two parties trapped in the tunnel the only noise Izuku was able to produce, with some effort, was a high pitched, vibrato, screeching from the end that facing the other end of the underpass.

Luckily the third party of this peculiar circumstance, instead of the being meathead some other heroes might assume him to be and punching away this problem, All Might inferred that there was more to this situation than he knew and decided some information gathering wouldn't go amiss.

"IF YOU CAN UNDERSTAND ME, CEASE WRITHING!"

Izuku immediately ceased writhing.

Perplexed by the sudden compliance of the, apparently sentient, creature blocking the tunnel, All might deduced that this may be the result of quirk usage gone awry and maintained his heroic composure.

"MAY I PUT MY HANDS UPON YOUR PERSON?"

A nearby calloused protrusion, that could, charitably, be called a tail if it weren't so stubby and tipped with a large, fang-like extension, wiggled slightly as if to say "Please help."

Taking that as confirmation All might took hold of another fang-like off shoot from the ground in one hand and slid his other hand underneath to ease the seal of air pressure and friction trapping the poor citizen within the confines of the underpass.

"IF YOU CAN MAKE YOURSELF ANY THINNER, PLEASE DO SO. THIS MAY FEEL SOMEWHAT UNCOMFORTABLE BUT I'M GOING TO TRY AND PULL YOU OUT. IF YOU WISH ME TO STOP, WIGGLE YOUR TAIL."

All Might is very charitable hero.

6 and a half minutes of activity (Not unlike pulling an overfilled full garbage bag out of a plastic bin by pulling slowly from side to side) later and Izuku, as well as the villain who had left a long green stain on the wall, had been freed from the tunnel, even if the villain only had a short while of freedom before being ensconced by a pair of soda bottles courtesy of All might's groceries.

Izuku had finally been able to open his eyes, of which he had several (allaying his earlier worries of not having any). The 7 or so on his front were having trouble staying open to focus on All Might, whatever adrenaline that had been keeping the pain away had drained and left him very sore and heavily fatigued.

Slowly losing a battle against unconsciousness, the last thing Izuku saw before fading was All Might's concerned face, haloed by copious amounts of steam.