Messi went down to the pitch once the game was over to celebrate with his team-mates and gave tactical instructions to several players. He and Ronald Koeman, in fact, were quite clearly coaching the players through extra-time.
Once it finished and the tie went to penalties, Messi was also one the grass with everyone except the eleven still on the pitch, supporting his team-mates as they took their spot-kicks.
Messi has been criticised heavily throughout his career for his quiet personality and apparent lack of ability to take control of shaky situations and offer leadership and inspiration to his less-experienced colleagues. His activity yesterday disproved that, underlining his ability to lead.