He and his Leicester team-mate Islam Slimani were both dropped for the second of Algeria's World Cup qualifying defeats last week against Cameroon with Mahrez having transferred his disappointing club form on to the worldwide stage, but here was an opportunity to show why he was voted the best player in the division by his peers just 18 months ago.
Hal Robson-Kanu and James Morrison sat out with muscle strains and there was no place in the squad for £15m man Oliver Burke.
Chadli, meanwhile, started his first league game of the season wide left after receiving an injection in his back last week.
It was Simpson who tested Boaz Myhill with a shot of his own as the clock ticked towards the interval, a powerful effort from outside the area forcing the Welshman to beat the ball away to his right.
35 - MYHILL SAVE - Fuchs sends in a long throw that Hegazi glances away.
Immediately going up the other end, West Brom almost got in as some good passing and movement enabled Gareth Barry to set Jay Rodriguez clear beyond the home defence, only for Harry Maguire to produce some excellent recovery defending and out-muscle the forward from the ball.
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Vardy, who had been quiet since his earlier burst, had a header at the back post before half-time but he exploded into the second half. A sloppy back-pass from Hegazi was seized upon by the England striker and his touch took the ball beyond Myhill before the pair collided just outside the box.
He should have been picking the ball out of his net moments later when Mahrez blasted over a golden chance from 10 yards out.
That chance sparked the home crowd into life after a boring first half.
But just as the locals were starting to sense a goal, Chadli silenced them with a free-kick of clinical precision.
Leicester went into the match just two points off the foot of the table and without a Premier League win at home against the Baggies, but they looked the more likely to find the breakthrough in a dreary first hour.
Meanwhile, Craig Shakespeare thought his Foxes side should have taken the lead, he told BBC Sport: "When you have as much possession as we did and then go a goal down you are always anxious how it affects the players but we showed the spirit we have and the resilience and we deserved a goal".
Iheanacho was yellow-carded for a late challenge on Krychowiak and Chadli stepped up to bend the resulting free-kick over the Leicester wall and past a wrong-footed Kasper Schmeichel. Subs: McClean 7, McAuley 6.