It can be flipped with Salah going from decent Serie A player to best player in the EPL. Sometimes players fail not because of the league but the team dynamics and environment. Lukaku scored a lot in England, at one point he was rivalling Harry Kane, he also scored at the same rate in both leagues. This gets forgotten for some bizarre reason, Lukaku had 25 goals one season in England.
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Pogba was never incredible at Juventus, he had a great highlight reel but also he was surrounded by a fantastic set of midfielders which limited his flaws from being highlighted. The PogBoom youtube videos didn't show how inconsistent he was or how he was considered the worst number 10 in their history when they tried to shoehorn him there because like here, they weren't sure where to get the best out of him.
Where should United sign from if we can just disregard a top 5 league like Serie A