Manchester United didn't do well in the beginning of the season last year and they are doing the same this season. In the last two games they lost unexpectedly vs Crystal Palace in EPL and Galatasaray in Champions League. Defensive errors are unforgivable in such highly intense competitions and injuries couldn't be an excuse there.
However, Man United's performance at the start of this season was quite disappointing. They may not be a bad club, but, they sometimes play very badly. Even when they have good opportunities, they cannot execute them well enough. Some of Man United's failures in previous matches were also quite surprising, and Ten Hag is someone who is quite responsible for that.