I think Mannah's value to side is underestimated. Yeah he was poor last night, yeah he isn't the greatest, yeah there are better.
But his value lies in the ability to pretty much make 100 or so metres, 30 or so tackles and very few errors in about 45 minutes each week. And on a average wage ( probably around $350k ). There is very little difference between his best and worse which is what coaches like, they know what they will get.
The first 20 is normally the fast / hardest part of the game, which is what Mannah plays in. And while there are other props ( like Paulo and vava) who attack a lot better, Mannah's value is his defence / error rate, how often do we see Mannah miss a tackle ? Drop a ball? Give away a penalty ? very rarely and this is extremely important in the first 20 when the game is still an "arm wrestle".