You're quite right that it can never be proven that he got it wrong on Sunday. But again, I'll go back to it, I don't understand why people assume you can only discuss things if you can offer tangible and irrefutable scholarly evidence that what you're saying is correct. I'm not going back to University and doing a thesis, I'm discussing football on a football forum.
It's sometimes like having a conversation with Sheldon on The Big Bang Theory with some people on here (I'm not directing that at you in particular). We're discussing football, a game regularly described as "all about opinions". Sometimes my opinion differs to the manager, the CEO, the players, whoever. Some people are too insecure to disagree with a an expert, or don't dare offer an opinion without scholarly referenced research. Good for them, but personally I'd rather have an opinion, and I'm happy to listen to other people's opinion, that's why I come on a football forum.
Your second point that "the only judgement of Pellegrini can be at the end of the season if we win the league or not" I completely disagree with. That's the only judgement of whether he's reached his target, but there are many other judgements to be made along the way. I'm happy to give him credit when he does very well, as I did after the Sevilla game. I'll also criticise him when he gets it wrong. He got it wrong on Sunday, and even if he wins the league, that won't change that.