The 35-year-old musician - who is nicknamed VMan - is good pals with the 'Wait and Bleed' hitmakers' guitarist Jim Root, and when the star asked him if he knew a good bassist the guitar technician couldn't pass up the opportunity to join the heavy metal outfit.
He said: "Me and Jim became friends while I was teching.
"He was asking if I knew any bass players. When I found out what for, I put my hand up right away.
"He pointed out, 'But you don't play bass?' and I said something to the effect that I could do whatever he needed from me. Then I just had to make sure it was true."
Alessandro had to borrow a bass guitar from Mastodon's Troy Sanders for his audition and then crammed in as much learning as he could, before trying to impress the band at the try out by playing the most complex tunes he had taught himself.
Slipknot's former bassist was the late Paul Gray, who was found dead in a hotel room in May 2010 aged 38.
Alessandro insists he was never trying to replace Paul in the band, but he has learnt a lot from the late star's bass playing.
He added to Bass Guitar magazine: "My approach isn't the same as Paul's. I can't be him and never will be, every player is ultimately born different.
"That said, if you listen to Paul's note choices on 'Vermilion', he was all over the shop and it sounded great. I wanted to try things like that.
"After listening to his stems, I honestly looked at bass in a different light and understood how to support everything as the backbone.
"Take the bass out of the mix and everything will fall flat on its a**e - and equally, if you mix too bass-heavy, you're not going to get your point across either.
"Lead guitarists, on the other hand, always need to cut through because that's what the job entails. As the rhythm section, we're there to hold the fort."