You're combining two different seasons into one here. 19/20 and 20/21.
19/20 was his great season. One of the best players in the league for the first half of that season and looked like he was on the verge of stepping up to being a truly world class player. Then the back injury happened in January (a bit before that but he played through it which in hindsight was an absolute disaster of a decision) and he's never been remotely close to that level again. He'll have the odd game here and there, but not in terms of consistency. We then had the football lockdown while he was injured, which is why he actually played again that season as he was fit by the time the lockdown ended in June. The interesting thing when he returned was that, while he wasn't performing to the individual level that he had been previously, he was actually showing a level of vision and passing ability that he'd never really showed before. It actually gave me a lot of hope that he'd spent his time off improving that part of his game, and if he combined that with the individual brilliance that he'd had before we'd be looking at at a truly great player. Unfortunately that didn't happen and we basically ended up with the worst of both worlds instead of the best.
20/21 is, as I said in the other post, the season that he still got a bunch of goals and assists but his overall performance level dropped massively. There were games after games where he'd be absolutely stinking up the place and one of the worst players on the pitch, if not the worst, but he'd be in the right place to bundle one into the back of the net at some point. Him getting those goals meant plenty of people still talked about him as if he was playing great and they couldn't understand why others were criticising him, but it was obvious that it couldn't continue. Either he'd kick back into form, or the goals would dry up. He then went to the Euro's at the end of this season and yes, the end of that may have been a contributing factor to why he declined even further in 21/22 where he was truly atrocious and ultimately got rightfully dropped by Rangnick.
I did note the lack of a proper break which is why I wasn't fully in the 'sell Rashford' camp that some were. I also wanted him to get a chance under a proper manager who would hopefully create a system for our attacking players to work in instead of being expected to do everything individually. However I wouldn't say I was confident and I also would have fully understood if ETH had decided to sell him, as he hasn't just been playing poorly over these last few years but he's also been incredibly lazy as well.
This season he's been very up and down. Same games where he looks no better than he did last season, some where he's looked more like 20/21, and some where he's been playing quite well. Not 19/20 good, but still good. It is worth noting that most of his truly bad performances have come either on the right or the early part of the season when he was playing at striker. His more recent striker performances have been a bit better, and most importantly his performances on the left have been where he's actually had quite a few good performances. It's just unfortunate that due to Martial's injuries and Ronaldo not being good enough to deserve more starts, we haven't been able to just get a run of games on the left. I'm hopeful that once that finally happens we might see a more consistent and quality Rashford, which is why I really do hope we sign a striker in January.