The Sheffield Wednesday striker was sent off for the second time in 2016 playing against the Lancashire team with his 65th-minute red card.
Most of Hillsborough’s 24,843 crowd – and referee Scott Duncan – missed the off-the-ball incident with Ben Pearson.
But after consultation with his linesman, following Preston protests, Duncan sent off Forestieri, who had given the Owls a first-half lead with a deft header.
But that was only the start of the drama, as the 10-man hosts increased their advantage through Steven Fletcher’s penalty before a crazy finale.
After Eoin Doyle made it 2-1 – when the Owls were down to nine men after an injury to Will Buckley – the former Chesterfield striker and Jermaine Beckford were sent off for fighting each other.
It defused the pressure building on Wednesday’s defence and allowed Carlos Carvalhal’s side to hang on and climb into the top six.
But victory came at a cost. Captain Glenn Loovens and winger Ross Wallace limped off before half-time, and Buckley was forced to finish the game at pedestrian pace with a shoulder injury with Wednesday having made all three substitutions.
Forestieri will serve a three-match ban – missing games with Reading plus the South Yorkshire derbies against Barnsley and Rotherham United – having netted in his last three outings.
“I didn’t see anything with Fernando, I just saw their players running towards the linesman complaining,” said Owls defender Tom Lees.
“It’s one of those things; if you give someone the opportunity to try and get you sent off, then they are probably within their rights to get you sent off.
“Any advantage a team get they will take, you can’t give them an opportunity.
“Losing Fernando is a massive blow, he’s just come back into form.
“He’s a fantastic player, you see he can win games on his own.”
The Owls are also without Gary Hooper (hamstring) for several weeks, and Portuguese striker Lucas Joao is just returning from a long injury lay-off.
Lees said: “It will be a blow but the positive is we have a lot of good options up front. Bringing on Steven Fletcher (as a substitute) in the Championship is probably unheard of.
“I am a big fan of Lucas Joao, too, and hopefully we can get him fit and firing on all cylinders.
“When he is fit, he is unplayable.”
The red cards overshadowed what had been a good display from Wednesday in the opening 45 minutes.
Winger Adam Reach, playing at left-back, was key against his former club, picking out both Forestieri and Joao but neither striker could beat Preston goalkeeper Chris Maxwell.
The summer signing from Middlesbrough played a big part in Wednesday’s opening goal on nine minutes. His shot was going wide until Forestieri diverted it into the net with his head for his sixth goal of the campaign.
Callum Robinson replied for Preston, his long-range effort smashing against a post.
The Owls should have been 2-0 up just before the break when Buckley somehow spurned two golden chances.
He broke clear but, with only the goalkeeper to beat, opted to pass to Forestieri but his ball was poor.
Then, when Forestieri crossed back into the area, Buckley was free at the back post, but he headed wide.
Forestieri’s red card seemed to rally the Owls, feeling a perceived sense of injustice, and they made it 2-0 when Maxwell clattered into substitute Buckley and Fletcher netted from the spot.
With Buckley off the pitch receiving treatment, Doyle gave Preston a lifeline, before he saw red after his bust-up with team-mate Beckford.
“It was a pretty crazy game,” admitted Lees.
“A shame really because we have been under pressure the last few weeks, especially at home, and I thought we started the game really well.
“We tried to be positive, closed the opposition down really well, but all the carry-ons in the second half spoiled the game.
“It was scrappy, and to be fair, we were lucky when their incident happened as it helped us, relieved the pressure.
“You can’t be throwing punches. The ref saw it and had to make a decision.
“I have only seen anything like that on TV, I remember an incident with Lee Bowyer and Kieron Dyer (at Newcastle United). But it probably helped us out a bit,” added Lees, who was a team-mate of Beckford at Leeds United.
“It was his passion to try and get the next goal.
“There was a disagreement on whether the ball should have been passed, but he’s a really good guy and these things do happen.
“You don’t want to see that, but it took the sting out of the game and helped us out.”