Firstly, no-one would offer an argument for less goals.
No-one has a very good idea what Kagawa would be able to offer there because the fecker hasn't been given a chance. That "it wouldn't be enough" is obviously just your presumption based on the limited amount of time he's been played at United, mostly at wide-left, which can't be strong reasoning.
We'd probably lose some goals from Rooney, but obviously that doesn't matter if those lost goals are made up by Kagawa himself, along with the goals that would be up for grabs from the more creative football we presume could be played with Kagawa at #10.
It's absolutely stupid not to give him a run of games there, at least. It's what he was bought for.