#2 Thomas Vermaelen

The Belgian defender has been so unlucky with injuries since joining the club from Arsenal. He's managed only 19 league appearances in 3 seasons, never getting the chance to show his best ability.
It must be mentioned how well Vermaelen played when called upon against Real Madrid in December in the 3-0 win at the Santiago Bernabeu; he was solid and comfortable beside Gerard Pique, restricting Madrid's attackers to poor shooting opportunities. There is a world-class defender in Vermaelen but, at 32, his time may have gone.
Yerry Mina has arrived in January from Palmeiras to challenge for the centre-back position so Vermaelen has even more competition. Barcelona just let Javier Mascherano go, their legendary centre-back, because of his age and declining quality, so it seems obvious that Vermaelen will follow suit.