First of all he wasnt that good either way in Manchester United when he played. Yeah he looked pretty decent on Dortmund but it wasnt that significant on UCL either so the quality of the league totally is a factor to consider if you wanna speak about quality. Aside from that, Ten Hag clearly stated that he doesnt want a player to be showing half ass results in training and clearly Sancho was showing that half ass result so yeah he was suspended after he made that one sided clarification on X.

tldr version of their problem was that Sancho totally shows no respect for him thus he was suspended