Their hosts have won the European Champions Cup for the last three years running but they were comprehensively beaten by Wasps earlier in this year's competition.
And Horgan, who played his entire professional career at Leinster as well as winning 65 Ireland caps, expects fly-half Jonathan Sexton to play a pivotal role.
"You have to starve them of possession," he told The 42. "Toulon are used to being dominant in the French league and in Europe.
"Leinster don't necessarily need to be dominant in possession but need to be almost breaking 50-50.
"I think we have seen indications that Leinster are looking to move the ball outside of the ten with regards to forward carriers and I think that's probably a smart move against Toulon.
"Toulon don't give you a huge amount of opportunities but if you do turn the ball over, they can be disorganised because they are such an attack-minded team. And you have to be prepared to capitalise on that.
"[Sexton] will certainly be alert to any opportunity. He is a risk-taker and if something is on he is always prepared to make the right decision."
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