Yes, I'm sure you could turn a Barcelona side that had finished third behind Real and Villareal the previous season (almost 10 points behind Villarreal too and 17 behind Real ) into the best side in Europe. That too without any major signings.
I'm sure Rikjaard before him just chose not to the previous season.
Well, excuse me but it was not the same, Ronaldinho was the problem that year he stopped playing and started going clubbing and destroying the atmosphere in the team..Moreover, when one team win everything it is very difficult to motivate them to win everything again the next year, that is why no team have won the Champions League two years in a row, even not Barca...What is more, you can absolutely say the same about Rijkaard when he took the job because Real were dominating like hell and then after Martino, Luis Enrique took the job and won the treble. In 2012-2013 Barcelona became champions and were unlucky to be eliminated by Chelsea in the semi finals of the Champions League even without a head coach. I repeat without a head coach. There are only two-three differences between Guardiola's and Rijkaard teams- Dani Alves, Yaya Toure and Henry and then Pedro and Villa, the core of that team was absolutely the same. This Barca team are playing together and are taught how to play together since a very early age. That is how their academy works. His tactical decisions most of the time, especially when he decided to change that 4-3-3 the Barcelona team always used and have been using all the time ( even in their academy) he started his downfall. The tactical decisions were totally strange to say the least. That continued with his Bayern team that was thrashing everything on their way and then were totally outplayed and outsmarted in most of the important games they had to play in his reign. That is just my opinion on the matter.