United registered a dismal first-half display as Bournemouth took the lead twice through Dominic Solanke and Justin Kluivert.
Bruno Fernandes scored both of United’s equalisers, one of which came from the pantly spot, but the draw leaves Ten Hag’s side with just one win in seven Premier League games.
Garnacho, meanwhile, struggled to make an impact in the first half and was replaced by Amad Diallo at the break with United looking to score an equaliser.
Speaking after his side’s draw, Ten Hag highlighted the problem United had on their right flank before deciding to substitute Garnacho.
Asked about the issue on United’s right side for Kluivert’s goal, Ten Hag said: ‘It already happened before that.
‘That [Dominic] Solanke was dropping in, we didn’t anticipate, something to do with our organisation, that is something I point to, you need some coaching there then you stop it at the source.
‘In the second phase it was too easy, on the right side there was a big gap and Kluivert should never get that ball there but if [he does] then we have to close it, that’s clear and obvious.’
Asked if that was the reason why Garnacho was substituted, Ten Hag replied: ‘I think we had to repair over that right side.
‘We didn’t play well, the spaces were there in possession, we had to bring a sub there.
‘Garnacho was not training during the week, only yesterday, we thought bring some energy, bring some more quality, quality in the sense of co-operation, togetherness over that right side.’
In his interview to MUTV, Ten Hag added: ‘We conceded two unnecessary goals, we twice lose balls in situations you can’t lose them, I think three times even, that is one part of the story.
‘The other part is that right side was so open, it was too easy for them to create chances.’
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