Barcelona is not a team that has never won the La Liga trophy, they have players who have above average abilities, winning the La Liga trophy is not something difficult for Barcelona, especially in the previous two seasons they had celebrated the success of winning the competition with Xavi Hernandez.
Every team have strength and weakness and a league champion does not need to be a perfect team. They only need to be a best team in that season, with better performances and more total points than other competitors. In golden era of Barcelona years ago with Messi, Iniesta, Xavi, Villa, Alves and more star players, they actually had weakness but their strength helped to cover most of their weakness.
The new players brought in do not guarantee that Barcelona will be able to win trophies, but with increasing strength it will open up opportunities for Barcelona to achieve success. Last season Barcelona performance was not too bad, they only lost out to Real Madrid in the title race.
Recent seasons, they had good performances but did not play better than other competitors like Real Madrid, Atletico Madrid except two season ago when they won La Liga.
In this summer and even months before that, Real Madrid have good preparation for their squad and Barcelona, if want to beat Real Madrid in La Liga, not only in direct matches, but in a long season, need to improve their squad a lot.