Hmm, while Manu playing better obviously will affect his plus/minus, I don't understand how you connect that to 'the bench struggling'.
And the reason is that Manu has actually played the most of his minutes with starters.
w/ Tim and Tony (474:59)
w/ Diaw-Parker (327:12)
w/ Diaw-Splitter (308:53)
w/ Duncan-Leonard (289:15)
http://www.basketball-reference.com/.../lineups/2013/
There's actually not one single top-10 (minutes) 5-man lineup he's been on that doesn't include at least one starter. (this is actually easier to see on the 3-man lineups listing)