Joshua was outclassed and comfortably beaten by Usyk when they first met last year.
The heavyweight rivals faced off again earlier this month and AJ gave a better account of himself the second time around.
The bout was much closer on the cards and Joshua had Usyk reeling in the ninth round following a flurry of body shots.
But as Usyk often does, he came out firing in the final three rounds to clinch the victory on the scorecards.
Joshua would have fancied himself to stop Usyk in the tenth round, but the Ukrainian's punch output was ridiculous as the whole complexion of the bout changed.
In the lead-up to this bout, Joshua altered his team as Garcia and Angel Fernandes replaced long-term trainer Robert McCracken.
Garcia has now acknowledged that AJ was "relying on a good shot" in the final rounds to get the win:
"Joshua was mentally defeated since round ten or eleven," Garcia said to Izquierdazo (via RingNews24).
"Round ten was a big one for Usyk. That changed the whole fight and mentally we were defeated.
"At that point, we were relying on a good shot or something like that, because Anthony is a hard puncher, and one shot can change a fight.
"That is what we were hoping for, one shot, because he was dominated at that point of the fight."
Garcia went onto suggest that they felt they "had the fight" in their hands before Usyk came on strong in the championship rounds:
"The fight was going well for us, but the tenth round changed everything. The last three rounds of the fight were the ones that changed the fight. In the ninth round we were close to winning, perhaps even by knockout.
"Through round nine, we were winning the fight on the scorecards, but it was decided in the last rounds. Usyk is a great fighter, with a tremendous heart, who knows how to finish strong, like champions do.
"During that ninth round, me and the guys in the corner were telling ourselves that we had the fight. But nothing is secured until the final bell. Usyk came back stronger in the tenth round and that changed the fight.
"I have even thought in these last few days that it was Anthony Joshua who mentally defeated himself. In the ninth, Joshua threw a lot of punches and almost put Usyk out.
"Maybe he emptied [his tank], but maybe it was just that Usyk came stronger and more motivated to the tenth round, we don't know.
"In this fight, I think that Usyk was mentally stronger. I think he was stronger, to the point that after being close to a knockout, instead of giving up, he came back stronger.
"His will and his desire to demonstrate to his country that he couldn't let himself lose the fight, made him come back."