The thing is with Mulumbu, he seems to have gotten older with age when he should have reached his peak by now (I think he's 28 years old).
In his first Premier League season under Di Matteo he was genuinely class, 7 goals and loads of great defensive performances with high energy. The next few seasons weren't as good, but mostly because it would have been hard to match such quality. Then after a few more seasons he became very average and wasn't on cheap wages either.
It's a shame, it's almost like his style of play has lead him to burn out earlier as a player. You could never question his desire but he seems to unfortunately be one of those players who declines earlier than most, because there's no way Dorrans should be ahead of him.