I don't see this about 'attitude' at all myself. The whole team has suffered from losing Sagna, Debuchy, Koscielny and Arteta, but I think Mertesacker is probably the one who individually has suffered the most.
Firstly, Chambers is NOT a good RB defensively. He is too often positionally wrong, he is at best mediocre 1v1 against pacy opponents, and he hardly ever stops a cross from coming in. IMO Mertesacker has because of this already been dragged wide right more times this season than was the case for all of last season. This is all playing to Mertesacker's weaknesses the way I see it. Say what you will about Jenkinson, but he rarely left Mertesacker exposed like that.
Secondly, having Monreal instead of Koscielny next to him on the other side has also been a massive negative for Mertesacker. To start off with, Monreal had to learn the job on the job, so to speak. I think Monreal has gradually improved in that CB spot, but he certainly doesn't help Mertesacker the way Koscielny does. On the surface, I can understand Wenger's thinking behind putting Monreal at CB; he has decent pace for a CB, he reads the game well, and is quite proactive in his play. These are the same qualities that Koscielny has in abundance. Problem here is that Monreal is nowhere near as good at it as Koscielny. His aerial ability is mediocre. He is nowhere near as fast as Koscielny, so he can't cover behind Mertesacker as well as Koscielny when needed. He is not as physically strong as Koscielny, so he loses out in physical duels more than Koscielny does. He is also not as good as Koscielny at getting in front of the attacker to steal the ball away. In short, this means that Mertesacker ends up in a whole bunch more defensive situations than when he plays with Koscielny.
FWIW, I don't quite buy into this argument I have seen pop up here that Koscielny has papered over Mertesacker's weaknesses. IMO they really COMPLIMENT each other. I think Koscielny benefits a whole lot from having Mertesacker next to him. Koscielny is obviously the better player; that is obvious. Koscielny's playing style is quite adventurous though, and Mertesacker has been behind him to mop up on countless occasions. Mertesacker does that due to his excellent reading of the game. Of course, Koscielny does the same for Mertesacker.