Australia fly out for Bangladesh on Friday for their first Test tour of the country since 2006. While they are wary of the threat posed by Bangladesh in their home conditions, Smith's men will also take heart from their own fighting performances in India earlier this year.
During that series, Khawaja was overlooked for Shaun Marsh, while Agar sat out and fellow left-arm spinner Steve O'Keefe was used, and collected 12 wickets in Australia's surprise victory in the first Test in Pune. However, O'Keefe is now out of favour with selectors and Marsh has also been left out of the squad for the Bangladesh series.
Should he win a recall as expected, Khawaja will need to find a more effective way to handle spin than he has on previous tours of Asia. In all, he has played 15 first-class innings in Asia - all in India and Sri Lanka - and has passed fifty only once, when he retired on 101 in a tour game in Colombo on the 2011 tour of Sri Lanka.
"I think Usman's going to be a really big player for us this summer," Smith told reporters in Darwin on Friday, ahead of Australia's departure for Dhaka. "He's done incredibly well in Australia over the last couple of years. He hasn't played any cricket [since January], so he's chomping at the bit to get out there and I dare say he'll get his opportunity."
Agar has not played Test cricket since his surprise debut series, the 2013 Ashes in England, when at the age of 19 he made a memorable 98 batting at No.11 on debut in Nottingham. Agar had limited impact with the ball during that series, but has returned to favour and looks set to be the second frontline spinner behind Nathan Lyon, instead of uncapped legspinner Mitchell Swepson.
"I'd say so, at this point," Smith said when asked if Agar would be the likely second spinner. "He's improved a lot in the last couple of years. He bowled really well up here this week. He's probably going to get his opportunity first up, if that's the way we go.
"For me with Ashton it's when he's consistent and hitting a good length, he's going to be dangerous. I think that's the key to bowling spin, particularly in the subcontinent. I think he's gained a lot of confidence in the last couple of years. He understands what he's trying to do now a lot more. If he can keep being consistent and hitting good lengths, I'm sure he's going to be a big player for us on this tour and in the future as well."
Smith also agreed that Agar, who is now 23 and has two first-class centuries to his name, could even find himself as a top-six Test batsman at some stage in his career.
"I think he's a real talent," Smith said. "We've seen him play some really good innings - obviously his debut, where he scored 98, was quite phenomenal at number 11. He's got a lot of potential with the bat and he's a great fieldsman as well. He's a good package and an exciting player for the future."
Australia's other selection decision will be whether to retain Glenn Maxwell at No. 6 or opt for the medium-pace allrounder Hilton Cartwright, with only two specialist fast men likely to be in the attack. Maxwell should have the front-running after his strong display in India earlier this year, where he made his maiden Test century.
"I thought he performed really well in the last two Test matches in India," Smith said. "We'll wait and see what the conditions look like, and what we feel the best make-up for the team is when we're over there."
Australia are conscious that recent wet weather in Bangladesh could yet affect their warm-up game in Fatullah, which would leave them short of any match practice in the country ahead of the first Test, which starts on August 27. Australia's squad members played a three-day practice game in Darwin this week, but Smith is well aware that Bangladesh could be a tricky proposition at home.
"They're a very good side in their own conditions and they're going to be very challenging," Smith said. "It will be great for us to get some game time there and play in those conditions, and see what we've learnt from India. Hopefully the guys have learnt a lot, and we're able to take that into this tour and have a really successful one."