Barcelona won two Champions Leagues under Pep Guardiola only because of Lionel Messi's brilliance, according to former Bayern Munich boss Felix Magath.
The Catalans won their maiden treble of La Liga, Copa del Rey and Champions League trophies in 2008-09, which was Guardiola's first season in charge.
They conquered Europe again two years later, again beating Manchester United in the final, to mark one of the finest periods in the club's history.
Guardiola, who left in 2012 after winning 14 major trophies, has been unable to win Europe's top club competition since.
He won a trio of Bundesliga titles with Bayern but suffered three consecutive semi-final defeats in the Champions League, to Real Madrid, Barca and Atletico Madrid.
Similarly, he has enjoyed huge domestic success with Manchester City, winning two Premier League titles, one FA Cup and three EFL Cups, but he is yet to guide them beyond the quarter-finals in Europe.
Magath has called Guardiola's possession-based system into question, suggesting it worked at Barca "solely because of Lionel Messi, who can decide a game out of nowhere".
"Messi won the titles, not Guardiola," he told Sport Bild. "Without Messi, this system has never worked as successfully for Guardiola, otherwise he would have won the Champions League with Bayern or Manchester City long ago.
"Tiki-taka only works if you have players who are technically superior to their opponents. For the spectator, holding the ball, as I call it, is just boring and a top team doesn't really need it.
"In my opinion, Guardiola generally too often gets lost in trying to win a game in advance. That tactic often ends in wrong decisions, which prevent success."
City's Premier League title was wrestled away last season by Liverpool, who became champions of England for the first time in 30 years in manager Jurgen Klopp's fourth full season in charge. Magath believes the Reds' success is more down to shrewd business in the transfer market than Klopp's system, however.
"Jurgen Klopp made the system a success primarily thanks to his decisions on personnel, not because of tactics," he said.
"If Liverpool hadn't reached into the coffers and bought goalkeeper Alisson for €60 million (£55m/$70m) and a defender in Virgil van Dijk for €85m (£77m/$100m), this system would also likely not have been successful.
"In the past, I'd have called it counter-attacking; today, it is called 'Gegenpressing'."