Personally, I'd be disappointed if MAN was downgraded to the B787, especially given the larger B77W needs to be fostered on more flights at the moment.
In all honesty if they'd gone with the B787 you'd be all over here shouting how Qatar was investing in an act of faith with MAN by putting their latest piece of kit on the route ensuring passengers have the most modern experience and freshest air. Be honest now
Truth is Qatar throw any old equipment in, their "fleet planning" is interesting to say the least. A330-300 on Monday, A330-200 on Tuesday, A340-600 on Wednesday with a B787 on Thursday is not actually unknown at some stations.