The lively winger was a fans' favourite at Allianz Park, but Sarries' director of rugby McCall fears he could return to haunt them in Saturday's showdown as the English side seek a second consecutive Premiership and European double.
With 11 tries in 15 games, Strettle is likely to be Clermont's danger man at Murrayfield and McCall knows it.
"Strettle is playing outstandingly well at the moment, he was playing outstandingly well when he left here," he said of the 33-year-old, who departed in 2015.
"We didn't want him to go, but he asked to be released a year early to go and have an adventure in France, which we thought he deserved.
"He's a player we think very highly of and he's playing really well at the moment. I've stayed in touch with him from time to time, but I think he's still in the players' WhatsApp group."
But, joked McCall, Sarries could use that to their advantage, as he added: "They might be throwing him a couple of curve balls."
— Saracens Rugby Club (@Saracens) May 12, 2017
Sarries, who face Exeter Chiefs in the Premiership semi-finals next weekend, are eyeing a slice of history, knowing one more win will bring up their 100th in European competition.
Clermont have been beaten twice in the continental showpiece, but progressed from their group as winners this season having lost only once.
Director of rugby Franck Azema is anticipating a tough test and acknowledged his side will need to be on top of their game to compete.
"Saracens are a steamroller from the first to the last minute," he told Clermont's official website. "They have a very pragmatic way of playing, very well oiled, which everyone sticks to and which builds the scoreboard little by little.
"We have to be very vigilant and concentrated to limit the impact of their big ball carriers. We have to be efficient while making sure not to be too focused on those carriers and risk opening the space around them.
"Our defence has to exert a constant pressure on them to prevent them from unleashing a style which makes them difficult to beat."
The two sides last met at the last-four stage in 2014-15, with the Top 14 outfit claiming a 13-9 victory.
The injured Sean Maitland will miss out for Sarries, with Chris Wyles deputising, while Azema's side are without crocked duo Wesley Fofana and Noa Nakaitaci.