Barcelona getting knocked out of the Champions League in the Round of 16 is now a genuine possibility, and there are some real causes for concern.Barcelona suffered a 4-1 defeat at the hands of Paris Saint-Germain. This is the first time in their history that Barcelona have suffered consecutive defeats at the Camp Nou in the UEFA Champions League.Barcelona are third on the La Liga table, eight points off league leaders Atletico Madrid who also happen to have a game in hand. The Cules haven't fully found their feet after the implosion that happened at the club in the summer that saw various issues come to a head.Without further ado, let's take a look at the changes that Barcelona need to make before next season.5 changes Barcelona need to make before next season#5 Sell aging playersSergio BusquetsBarcelona need to have a more youth-centric system as they are currently a bit too reliant on their veterans. Five of their senior players are over 30-years-old and most are on the decline. Gerard Pique is 34, Sergio Busquets is 32, Lionel Messi is 33, Jordi Alba is 31 and Miralem Pjanic is 30.Sergi Roberto, Antoine Griezmann and Martin Braithwaite are all 29. Marc-Andre ter Stegen and Coutinho are 28. If you look at the list, it's easy to note that some of these players are not performing at the highest level. They earn a lot as wages too.Barcelona need to sell players before they can buy new ones. It's time to drop the axe on some of these players so they can kickstart a squad rebuild which is indeed the need of the hour.Sergio Busquets won just 1 of the 6 tackles he attempted against PSG.Miles off. 😬 pic.twitter.com/onDoEMno36— Statman Dave (@StatmanDave) February 16, 2021#4 Sign a strikerFC Barcelona v Valencia CF - La Liga SantanderLetting Luis Suarez go has backfired for Barcelona. The Uruguayan had an illustrious career and he was shown the exit door in the summer in rather disgraceful fashion. It caused a huge outcry and even Lionel Messi publicly expressed his disappointment at the club's actions.In the absence of Luis Suarez, Barcelona struggled to find goals, especially in the first half of the season. Martin Braithwaite has been nowhere near prolific and Griezmann took a while to hit his stride this season. Ronald Koeman had to rely on 18-year-old Ansu Fati for goals in the opening part of the season.Meanwhile, Atletico Madrid's title-charge has been powered by Luis Suarez's goals. Barcelona need to sell some of their players and free up funds. They are in desperate need of a striker who can score more than 20 goals a season.One of the best individual performances in the last decade of the Champions League. Luis Suárez conquered Paris that day. pic.twitter.com/yHMI39e9W8— Barça Universal (@BarcaUniversal) February 15, 2021