Messi is obviously the player you want to watch anytime he’s on the pitch. However, the person I think might have the most ability to determine Barca’s fate is their world-class German keeper, Marc-André ter Stegen. When the Catalonian side have been at their best, their keeper has been a large part of that. Barcelona have a lot of strengths, but their back four are not quite on the same level as their extremely solid midfield and their lethal attack. Still, when he’s at his best, ter Stegen is able to nullify many of their shortcomings. Just look at the UCL tie vs. Liverpool last year- in the first leg, a 3-0 win for Barca, MATS was phenomenal, turning away an egregious amount of very solid Liverpool strikes, making the final score far more lopsided than the game ever actually was. But back at Anfield, little lapses from the keeper such as a failure to hold onto the ball, allowing a second effort, allowing a well-struck but poorly-placed shot to sneak into the net, and of course, turning his back on the corner kick-taker, caused another evenly-played game to turn into a second rout, but with the roles reversed. You can’t win if you’re getting constantly scored on, but when ter Stegen is at his best, it’s extremely rare that Barca concede all that many goals.