Tottenham Hotspur are known for sacking their coaches either towards final of a competition or after the finals irrespective of performance. It didn't surprise me that Postecoglou has been shown the exit door despite winning for them the Europa trophy after all these years of going trophyless. They did same to Jose Mourinho when he was manager there, after taking them to the finals, just few days to the game he was sacked. Lol.
Yes, actually this must be quite surprising for Tottenham fans and also for all of us. Because Postecoglou has just managed to bring Tottenham to win the Europa League after a long time of not being able to get a trophy. So many people's expectations must have assumed that Postecoglou would be safe in his current position as a coach at Tottenham. But it turned out that the management said otherwise and finally fired the coach. I personally actually regret the decision made by Tottenham regarding this dismissal. But after I thought more deeply and found out what the cause was. It turned out that there was only one problem, namely Tottenham's poor performance in the Premier League. Because in essence the Premier League may be a priority for Tottenham, so the reason can be said to be quite reasonable too.
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Base on what you have said, I can only assume that the decision to sack him at the end of the season was already in place not minding if he wins the Europa league trophy or not. Else, they would have considered that he had won them one major trophy in Europe at a season when some of the major clubs in the EPL such as Manchester United, Manchester city, Arsenal, all went trophyless. I await to see who Tottenham would bring for Postecoglou replacement and what achievements he would bring to the club next season.