Double Rugby World Cup winner Cobus Reinach joined the Stormers at the start of the season and was their first choice scrum-half for the bulk of their United Rugby Championship (URC) and Investec Champions Cup campaigns with Khan and Stefan Ungerer sharing the duties as his back-up.
However, Reinach sustained a medial cruciate ligament injury during their 48-12 home victory over Glasgow Warriors in the latter rounds of the URC which meant Stormers director of rugby John Dobson then promoted Khan to the starting line-up for their subsequent matches.
The former SA U20 star wore the number nine jersey in the Stormers' final two league games of the season against Ulster and Cardiff and also started in their quarter-final victory over the Welsh region and semi-final defeat to Leinster.
The 22-year-old gave a good account of himself in all of those encounters and his impressive form was rewarded with a call-up to Springboks head coach Rassie Erasmus' extended 51-man training squad ahead of their clash with the Barbarians in Gqeberha on June 20 and next month's Nations Championship encounters against England, Scotland and Wales.
Mallett was appearing on the Talking Boks podcast with Brenden Nel and the show's host asked him if there were any players in that group who could be a surprise inclusion in a Springboks matchday squad and he replied: "Well, there's just some guys who've played some great rugby at the end of the season.
"I think Immy (Imad) Khan, who only started this year as a regular starter for (the Stormers) after Cobus Reinach.
"But Immy against Leinster was, I thought, had a great game and especially, particularly on defence.
"He made a tackle on (Sam) Prendergast that kept the Stormers in the game about five yards from the line, got off the line, tackled him back and really showed some good aggression.
"And young guys like this, it's always good to play. It's great to play them."
Mallett, who was South Africa's head coach from 1997 to 2000, is impressed with how Erasmus is building depth in the Springboks ranks and believes that by naming such a large squad, the current Boks boss is laying a solid platform for the future.
"This is classic Rassie. He covers every single corner, every single eventuality," he said. "I think we've seen already this year with the injuries that we've had, there hasn't been a sort of mass panic.
"If there's been someone like Sacha (Feinberg-Mngomezulu) going down or there's a problem around a tighthead prop.
"Because of the depth we've built over the last couple of seasons and he's certainly using this Barbarians game and the alignment camp as an opportunity to get as many of the youngsters in just to get them feeling comfortable about different environments.
"When you get into an international environment, it can be quite daunting if you're a young guy.
"And the earlier you do it, the next time that they might be selected will be maybe two or three years down the line.
"But by that time, they're going to feel comfortable in that environment and it's going to make them be able to perform a lot better."
Springbok loose forward Jean-Luc du Preez has firmly dismissed reports in France suggesting he is on the verge of retirement due to ongoing concussion-related issues.
The Springboks have called up Adre Smith to the national squad as further cover for lock for the Barbarians and the SA 'A' team's clash against Zimbabwe in Gqeberha on Saturday.
Players running out for the South Africa 'A' against Zimbabwe will forever be tied to the Republic unless they have parents or grandparents born in another nation.
Stormers director of rugby John Dobson has confirmed that the Cape Town side will not sign veteran Argentina lock Tomas Lavanini, and will instead back home-grown talent.
