Disclaimer: I’ve never played undertale (also i’m not a doctor, i should’ve put that on my last post as well)
Okay so if Mettaton is an android, his physiology is fundamentally different than a human beings. I’m going to be going about this with the assumption that Mettaton has the same general structure as a human body and the he experiences tactile sensations mostly the same way as a human being (ie. temperature and pressure register the same general way and that his joints work the same/similarly to a humans)
First- If he’s thrown into a well, while being sealed into a trunk in essentially the fetal position- Yes, he’s going to experience a great deal of lasting damage, especially if he has limited access to repairs.
The most obvious problem would be the denting. A human would bruise or break limbs, so depending on how tough his hull is he could deal with broken parts, loose wires, or a dented outer shell.
His knees could knock against his jaw, which could force him to bite his tongue (assuming that he has one for the purposes of speaking). The force of the knock could also fracture his jaw or his teeth.
If his arms are crossed over his chest, sandwiched between his chest or his knees, then he’s going to do some serious damage to his shoulders. Just from looking at the photos, it seems his arms are made up of interlocking segments. The place where the arm attaches to the shoulder would probably function kind of like a plug-in? and having his arms crossed in such a way would put a strain on the joint, which could end up slightly warped, which would reduce overall function.
If his arms are folded over his knees, so his knees are in between his arms and his chest, he would likely experience a similar problem, especially given he’d probably try to adjust the position of his arms. A repetitive rocking motion could cause a repetitive strain injury, and again, change the structure of the joint.
The big problem is his spine. The curled up position you’ve put him in is going to arch his back and expose his spine. Even a moderate fall could cause severe damage to ones spine, and being forced in that position for 6 months would damage his spine quite badly. the spine is made up of bones with a cushion of cartilage- even if he escaped damage from the fall, the cushion of cartilage would become compressed on one side, he could deal with nerves being pinched or crushed between the bones, and suffer from stress fractures from the awkward positioning.
Another issue would be his torso. In order for the torso to move the way it does, it’s made up of bones that connect with cartilage to offer it flexibility. Spending six months in a prone position could permanently warp or damage the torso.
Given that he’s an android, he wouldn’t have to deal with muscle atrophy, but the lubrication in his joints could become old/less effective which could leave him quite stiff.
And finally, in human bodies the longest nerve is the sciatic nerve, which goes through the hips and down to the feet. This nerve is fragile!! Having his hips titled in the way they would be could irritate or damage the nerve (again, assuming he has something similar)
And ofc you’d have to consider how he would deal with the isolation. Isolation of a period of longer than two weeks is considered torture, and can result in mental illness and ptsd, on top of the physical stress he’s under.
Really, it depends on how close to human you’re making his body. The important things to consider would be- where are his ‘bony’ bits, and what kind of strain are they under?
I hope this helps!! Let me know if there’s anything you need cleared up/ expanded on!