He made 11 changes to the line-up which played at Crewe just 48 hours earlier.
They booked their place thanks to first half goals by Josh Lelan in the sixth minute and Ibrahim Meite after 37 minutes.
“It’s a chance to win something. We came here with a good squad, we came here to play.
“First half we did great, second half we got a little bit lazy. Fair play to Hastings, they came back into it and we left it open for them.
We should’ve put them away, but we didn’t.
“It was a good run-out for the boys and I’m happy with the way the team played.”
Kewell praised the performances of several players, including Dannie Bulman - who he felt controlled the game - Josh Doherty and Yusuf
Reds will face National League South club Eastbourne Borough in the quarter-final in the New Year.