Khan has a future. He's a better fighter than Garcia. He was in control of the fight early on, but Garcia cleverly changed tactics in the third, targeting Khan's body. This rattled Khan who pressed forward in a bid to sieze back the initiative.
He spent (ever-so-slightly) longer engaged with Garcia in the subsequent exchanges (compared to his give and go missions of earlier rounds), and Garcia read this.
The shot that Garcia hit him with was a blind, timing shot, that misses 9 times out of ten. It changed the bout.
If they met again tomorrow, my money would be on Amir.
None of that is to diss Garcia, who is a powerful puncher with both hands, and a worthy champion.
I'm just saying that that punch came from virtually nowhere (as we've seen happen from time to time in boxing) and changed the complexion of the fight.
Having said that, Khan should have learned by now to keep his guard engaged, even when coming forward. He clearly will never learn that lesson, and will subsequently always be capable of beating, and losing to, anybody.