The career of Ronaldo Nazario is best defined by one key question: what could have been?Rightfully heralded as one of the finest players in history, the only real question mark hovering above the Brazilian icon is whether he could have become the greatest footballer the world has ever seen.And the reason that is a question in the first place surrounds the fact that Ronaldo was arguably robbed of his prime years by a series of serious injuries from which he never truly recovered.

Ronaldo's injury record

Now, it's important that we clarify that Ronaldo was still astonishingly good after his injuries, thriving at Real Madrid and becoming the greatest goalscorer in the history at the FIFA World Cup.

But he was always that extra five or ten percent behind the rampaging 'El fenomeno' that we saw during his PSV Eindhoven and Barcelona days, scoring 101 goals in just 106 appearances.

And although that mind-boggling form bled into his time with Inter Milan in 1997, it was at the San Siro where those dreaded knee problems robbed him of the explosiveness of his early years.

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Knee problems at Inter Milan

At the turn of the century, Ronaldo suffered back-to-back injuries that kept him away from the pitch for two years and today marks the anniversary of the second and arguably most heart-breaking.

That's because Ronaldo's relapse injury came just six minutes into his return from the first knee problem, which occurred in November 1999 and forced the Inter striker to undergo surgery.

And when he was given the go-ahead to return to action just five months later, Ronaldo indicated that he would be physically able to play around 15-20 minutes during a clash with Lazio.

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Video of Ronaldo's heartbreaking knock

However, in a moment that still makes for heartbreaking viewing after 21 years, Ronaldo suffered one of the most horrific knee injuries ever seen after attempting one of his trademark stepovers.

Lazio immediately kicked the ball out of play and the distress amongst players on both sides told you everything you need to know. You can watch the heart-shattering moment down below:

It just makes you feel so sad, doesn't it?

And just in case the gravity of Ronaldo's injury wasn't communicated in the video, then the account of physical therapist Nilton Petrone from a 2018 interview will do exactly that.

The full horror of Ronaldo's injury

Speaking to FourFourTwo, he explained: "When he returned for that match, he ripped the knee-cap tendon completely. His knee-cap actually exploded and it ended up in the middle of his thigh.

"If I showed you the photos I have from this case you wouldn’t believe it. Right after the surgery his knee was the size of a football ball.

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"There were three or four tubes draining the blood, it was something unbelievable. We had a few moments at the hospital when he was crying because he wanted morphine to stop the intensity of pain.

"One day in the middle of the night he called me and asked: “tell me I am going to be able to play football again, please don’t lie to me”. I was there, at the beginning of a rehabilitation when the whole world was saying that he wouldn’t be able to.

"The science said it was unlikely and the doctors had doubts. I had a unique experience and I thank God for having the opportunity to overcome that with him and that helped me a lot professionally."

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If anything, let that horrifying account bat away the question of 'what could have been?' with Ronaldo and make us all thankful that we ever got to see the Brazilian phenomenon play again.