Bobby Ryan doesn't play shutdown minutes but had a 44 CF% with Ceci and 56CF% without him. Really, it's most players on the team that see a drop in their possession with Ceci.
Hmm... maybe it has to do with how they are deployed together vs appart.
Ryan with Ceci; 44% OZ ratio. Ryan away from Ceci 54% OZ ratio.
Given how strictly Ceci got linematched, particularly on home ice, in the reg season, it's quite likely that Ryan faced far stiffer competition when with Ceci too.
That's the problem with WOWY stats (aside from the sample size issues when using a single year). They lack contextual information.
Also, pretty much everybody away Ceci is primarily playing with Karlsson, who just happens to be the best Dman in the league.
Edit: Worth noting, Pierre McGuire was asked about Ceci's performance in the playoffs, said he was good. Also mentioned that many of the alledged breakdowns were actually the center missing his assignment as the sens play a center lock system, so the guy you may have thought was Ceci's man was actually the centers.