21 is a significant number for Lionel Messi.
That's because 21 is the number of major honours the Argentinian has won during his time with Barcelona.
That includes six La Liga titles, three Champions League titles, two Copa del Rey titles and two Fifa Club World Cups.
Long story short, Messi has won everything he can possibly win at Barcelona.
So what is he still doing there?
He's already achieved everything he can possibly achieve at the Nou Camp.
He has become Barcelona's all-time top goalscorer, he has won four Ballon d'Or awards and proved time and time again that he is superior to Cristiano Ronaldo - at least he was.
This season, the player has struggled to cope with the demands bestowed on him at the La Liga outfit. He has struggled with injuries for the best part of six months, culminating in a serious hamstring strain that will keep him out until January.
As he continues to struggle, inevitable reports emerge linking him with a move away.
He quickly moved to dismiss those reports, declaring that he wanted to stay at Barcelona forever but is that really a good idea?
The 26-year-old has played himself in to a position where he could be considered one of the best players ever.
There are only two things missing. The first is a major international honour - a World Cup in other words - and the second is the proof that he can succeed away from La Liga.
Despite his bulging trophy cabinet, his career will never be well-rounded enough to be considered the best ever unless he wins titles in different countries with different clubs.
Imagine if he went to Premier League and inspired Arsenal or Chelsea or Manchester United to a title before moving to the Bundesliga to do the same. He would unquestionably be the best player ever.
At 26, he is arguably yet to hit his peak - incredible to think.
But at Barcelona, he has reached the pinnacle and, now, the only way is down for him.
Away from the club he could find a fresh challenge; something new to win, something new to prove.
He is supposed to be better than Ronaldo, but at least the famous Real Madrid player has won titles in the Premier League with Manchester United.
Unless Messi takes the chance and attempts to succeed away from the Nou Camp, he risks falling by the wayside as motivation fades and the admiration veers elsewhere.