Bayern Munich and Real Madrid are both regulars in this late stage of the Champions League, but both Roma and Liverpool beat the odds to make it through to this stage.
Heynckes came out of retirement in October after Bayern's faltering start to the season led to Carlo Ancelotti leaving as coach.
Real, meanwhile, sit fourth in La Liga, 15 points behind runaway leaders Barcelona, with their ongoing progression in the Champions League crucial in keeping coach Zinedine Zidane in a job.
Who has the edge? The fact Liverpool blew City away in the first leg, playing with such style and then when under a lot of pressure this week, stayed so composed for the win means they are the form team in my eyes.
Indeed, the Zimbabwean was part of the side which beat the Giallorossi on penalties in the 1984 European Cup final, so he is better placed than most to comment on what it takes to succeed in such a high-pressure tie.
Liverpool were paired against Roma after the Serie A giants performed an unthinkable comeback against Barcelona in the quarter-finals to go through on away goals.
That's despite the other semi-final seeing 12-time winners Real Madrid, who have won the Champions League in three of the last four years, face five-time champions Bayern Munich in a huge heavyweight showdown.
Salah will revisit the Stadio Olimpico on May 2 for the second leg of the semifinal tie.
Atletico, beaten finalists in the Champions League in 2014 and 2016, will still be strong favourites to win this semifinal, however. The teams also drew 0-0 at the Velodrome.
And even if Arsenal manage that, they then have to get past goalkeeper Jan Oblak, one of the world's best, to score the goals to take them into the final.
But the biggest challenge likely to face Arsenal is being able to match the work rate and tenacity of Atletico's high-pressing game.
"I still talk to most of the players there".