As the quarterfinal stage of the UEFA Champions League looms, many soccer experts have started their predictions on who might win the lucrative competition this season.Spanish giants Barcelona are among hot favourites to win the competition especially after their impressive 4-0 win against Italian giants AC Milan with four-time world player of the year Lionel Messi on high scoring form.After watching Barca’s game against AC Milan in the first leg at the San Siro and the second leg in Nou Camp, it was evident that Messi is the major threat in the Catalan side and his performance contributes to their chances of progression in Europe’s biggest club competition.Stopping Messi is one of the hardest assignments managers and defenders face when it comes to decisive games especially in the Champions League.Many defenders and midfielders have failed tasks against Messi.  AC Milan captain Massimo Ambrosini is one of the latest culprits to fall prey to the messiah trickery. During the second leg of the last 16 in the champions’ league, AC Milan entered the game with two goals advantage and with a mission to defend their lead.Skipper Ambrosini had been given primary mission to stop Messi with support from fellow Italian Riccardo Montolivo and French midfielder Mathieu Flamini.  Perhaps this was the area where Milan failed most against Barcelona because they lacked potent players to stop the Argentine.Amborosini lacks stamina to stop high flying stars likes Messi or Ronaldo. Messi is still the most dangerous player on the planet; he has an immense dribbling skill with high imaginative power. This is the reason why he has always emerged ahead of Cristiano Ronaldo when it comes to world player of the year accolade.One of the most important aspects with Messi is the fact that you have to be so careful with him when you possess the ball because Barca players react so quickly when they have lost the ball and fight to get the ball back. In doing so, they take advantage of those spaces you leave with incredible speed. At this point, one needs to be careful about Messi at all times. Normally when a team wins the ball, they think about attacking. Against Barcelona, you have to be attentive to Messi especially. Communication especially from defenders is vital to make sure there is always someone very near to him and this is where AC Milan failed. When Messi gets the ball, he is so swift and on most occasions has caught defenders off guard.  The hardest thing when you play against him is maintaining that level of intense concentration throughout the game. This is why you mark him with a younger midfielder or stopper than a veteran like Ambrosini.Teams with talented and youthful midfielders have performed so well against Barcelona. Real Madrid midfielder Sami Khedira has done so well against Messi. This has cemented his place in Real Madrid starting lineup and certainly strengthened his relationship with manager Jose Mourinho.Fellow Germany midfielder Bastian Schweinsteiger is another midfielder who has previously done so well against Messi especially during the 2010 world cup where Germany thumped Argentina 4-0. You expect Real Madrid and Bayern Munich to pose major threats against the Catalans in the later stages of this year’s Champions League than any other side remaining in the competition.Barca next play PSG in the quarter finals and manager Carlo Ancelotti should be consulting Mourinho on how to stop the little Argentine. A fit PSG side could perform better against Barcelona than AC MIlan.In the midfield, Thiago Motta and Blaise Matuidi should be given the responsibility to stop Messi and if they can do that, they would have a chance to cause an upset.