Uh... what exactly do you mean by "manual" all shifting is manual... as in the rider has to do it manually. Except for those fancy electronic prototypes. Or that crazy bike on late night tv.

The chain tensioner is designed for a frame that has vertical dropouts. Because you are unable to adjust the rear wheel position and thus unable to properly tension the chain... you would need to use that device.

If you are going to keep an 8 speed cassette on and you are saying that you would want to use the tensioner so that you could keep the chain way long and make it usefull in all 8 gears (as opposed to switching a cog everytime you wanted a different gear ratio) this is what the derailler was invented for.

I mean... you could do it... but why?