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I think Antony's extraordinary performance in La Liga is because besides the Spanish League is usually suitable for typical Brazilian players, on the other hand Antony already has a very good chemistry with Isco. In the end if Antony returns to Manchester United maybe we will see the old Antony again and we will miss the prime Antony at Real Betis. It seems difficult for Antony to develop at Old Trafford. Even Antony seems very happy at Betis in my opinion he has made a big contribution to Real Betis' performance.
The speculation you put forward is quite reasonable, where Anthony could lose his ability again when he returns to Old Trafford and plays for Manchester United, because at Manchester United he has never found a teammate who really fits him. And if that is the concern for Man United, then Man United should not miss the opportunity when Antony gets a high enough offer from another club, because besides Real Betis who are quite interested in being able to make Antony permanent, Bayern Munich are also quite interested in being able to bring Antony to the Allianz Arena.
When you play bad games in a row, you have to wait on the bench for a while. Neither the old nor the new manager put Onana on the bench. I think this decision is now damaging Manchester United. At least until the end of the year, Altay should be in goal. Manchester United should see how he plays when he is in goal. This could be an important move for next year's squad planning.
Man United has no more hope in the EPL, they are sure they will only finish in the middle of the standings. Therefore, maybe the coach can use the rest of their matches to do an experiment, including to give more trust to A, Bayindir to appear in the EPL match. This is quite important so that Man United can find the right strategy and line-up, so that Man United is really ready to face the next season. But regarding their match in the UEL, they must really perform well and seriously, because the UEL is their only hope to be able to win a trophy this season.