From the manual

You can appoint a team captain for each match. Experience and leadership are important abilities for the team captain, as the captain's level in those abilities give a bonus when calculating your team's total experience – which can prevent your team both from suffering confusion and from getting nervous in important and dramatic matches. In such situations, only the team with the lowest amount of experience at that particular moment in the match risks a negative event due to nervousness. The difference between the two teams' experience levels decides how nervous the players will get. Teams will never suffer from this in league matches or friendlies.

The team captain has to be in your starting lineup. If you haven't appointed a team captain, the players will choose a captain they think will do a good job.