Barcelona suffered a shock home defeat by newly promoted Alaves as manager Luis Enrique left Lionel Messi and Luis Suarez out of the starting XI.
Brazilian forward Deyverson silenced the Nou Camp when he stabbed home Kiko Femenia’s cross from seven yards.
Defender Jeremy Mathieu headed the equaliser from Neymar’s corner.
Ibai Gomez scored the winner – beating debutant keeper Jasper Cillessen from just inside the area – soon after Messi came on for the La Liga champions.
Suarez was introduced after Gomez’s goal but Alaves hung on to claim a first league win at the Nou Camp since 2000.
Barca are three points behind leaders Real Madrid, who beat Osasuna 5-2 earlier on Saturday in Ronaldo’s first game back from injury.
Enrique appeared to have one eye on Tuesday’s Champions League home group game against Celtic, making seven changes to the team that won 1-0 at Athletic Bilbao before the international break.
As well as forwards Messi and Suarez, midfielder Andres Iniesta started on the bench.
Although Javier Mascherano, Sergio Busquets and Neymar were included, Barcelona produced one of their poorest performances for some time.
Even after Mathieu’s equaliser in the first minute of the second half, Barcelona continued to live dangerously as Alaves’ Edgar Mendez had a goal ruled out for offside.
Mathieu blasted wide of an open net before Mascherano was caught out on the edge of the Barcelona penalty area and Gomez wriggled free to slot into the far corner.
Meanwhile, Pep Guardiola on Saturday extended his winning record against Mourinho to eight games in 17 matches after leading Manchester City to a 2-1 victory over Manchester United.
First-half goals from Kevin de Bruyne – a player Mourinho sold when he was at Chelsea to Wolfsburg – and Kelechi Iheanacho sent City into a 2-0 lead in the first-half before a colossal error by Claudio Bravo gifted Zlatan Ibrahimovic a goal late in the half.City held on to make it four wins from four and top the table while United slipped to their first defeat.
The friends turn foe have played 17 matches with Mourinho winning just three of the clashes.
