Yeah the 2014-2015 season but that wasn't down to any managerial tweaks from Pellegrini, He simply just gave Yaya a free role, no particular changes to his style of play, he just took the chains off.
Pellegrini's "managerial skills" actually revealed itself in his second season, as with most managers. I feel like that might be what we are witnessing with the dippers and Slot. The second season actually reveals the work the manager has done with the team.
I wouldn't judge him on his final season and there's a lot of debate if he laid down the tools due to the poorly timed announcement by Khaldoon and the boys from Catalonia, either way for me, Mancini > Pellegrini.