I imagine that there's all sorts to try and unpick in terms of RM's perceptions of his world and his circumstances.
He seems to hold a heightened view of his own ability - based on his undoubted natural ability, having this ability noticed by prestigious others (e.g. Rio) and having grown up in the Old Trafford environment.
I'm not suggesting that that has led to a sense of entitlement, but he's then experienced a number of failed moves and career setbacks, did he contribute to those in part? Quite possibly. Did he have too much expectation too soon, and did he expect to coast into those sides? Also possible.
The upshot, is that he seems to be left with little resilience to the setbacks your career might give you, and football needs bucket loads of resilience. Whenever a move doesn't work out, he appears to attribute that externally (i.e. it was due to someone else, or something else that it didn't work out, "it wasn't me - I'm decent, Rio rates me you know!") as opposed to reflecting on what more he could've done to improve things.
That Old Trafford mindset, a taste of the the big lights and the glory after growing up with nothing, really has hampered him, IMO.
I sense that he sees other career moves as a good opportunity, but that they feels a long way removed from OT life and that it's a long way back to the OT standard of living. Any further set backs (e.g. a loan at Cardiff not working), in this view, are even harder to take, as they feel like they are stalling his journey to get back to that level of glitz.
These experiences will also provide him with a really uncomfortable lack of congruence - "which is it meant to be, am I as talented as Rio says I am, or am I not good enough for Ostersund?" OT has basically set his standards too high. This final move, to the Lane was then the closest he'd been to getting back to that level - a Premier League club, a chance. By all accounts he impressed somewhat at first and he fit in well with the lads - but he always had a job on, to get to PL standard of fitness and to work his way into a side that was so settled and had played together for years - all whilst coming off the back of playing at a lower level in Sweden. All the while, we weren't hanging about - we needed to establish ourselves in the division. Frankly, we were growing faster than he could settle in.
That growth is nothing personal, but again - links back to that lack of congruence. "Which is it, am I talented, or am I not good enough to get a game for the Blades?"
And what seems to be the case, is that he takes the lack of opportunity personally, as a deliberate act against him, as though the club are putting him down. They aren't, it's more complex than that and linked to the form of others ahead of him. It's a shame, as others have said, that he then opts to vent those frustrations through thinly veiled social media posts.
I say GLTTL, but more in the sense that I hope he gets some help to understand his career path, to reflect on mindset, the opportunities available to him and to recognise what he could have - rather than pointlessly longing for what he used to have (OT) - unless he does that, nothing else will ever measure up and he'll experience more setbacks.