Rather than attempting to pan a stereo recorded piano to make it pseudo-Mono, why not just use, or create, a monaural acoustic piano sound. It will not suffer from the phase issues if you attempt to pan both sides of a stereo instrument to one channel.

You can use the "Monaural Grand MW" - this is one of the Factory Presets and shows how a Mono Piano differs...

This can be done quickly by selecting the Piano sound you want to use.
Then edit its Elements swapping the Stereo (St) Waveform for the corresponding Mono (Mn) Waveform of that same piano - if a mono version is provided. Both the Yamaha CF3 (9ft), and Yamaha S6 (6'11" ) acoustic grands come with a full compliment of Stereo and Mono piano Waveforms - both stretch tuned and non-stretched (flat) tuned.

For example, say you are using the “Full Concert Grand” - the Factory Preset is built from the “CF3 Stretch xxx St” set of Stereo Waveforms...

swap these for the “CF3 Stretch xxx Mn” set of Mono Waveforms... then STORE your new Mono Piano.
A Mono Piano will pan hard left or hard right better than a stereo piano

Hint: System Effects can be Stereo so often panning hard Tone side does not eliminate it from the opposite channel.

Wow, thanks for getting back to me so soon and on a Sunday! I actually looked for Piano on a quick search and couldn't find Mono Piano like my old S90es and gave up. I will try other method too. I forgot what Mn meant....

Thanks again!

From the HOME screen, press [CATEGORY]
With the Bank = All
and Attribute = All
In the box with the magnifying glass type in the letters "Monaural"

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I forgot what Mn meant....

It would be nice if there were some doc listing what all the abbreviations mean. (Maybe there is one?)
So while I'm here... what do the plus and minus signs indicate, anyone know? (I think I knew once...)

The plus (+) and minus (-) following the Waveform Name are different mappings of the same group of samples. This has an impact when combining Waveforms within an instrument Part.
+ waveforms have the samples shifted up in mapping
- waveforms have the samples shifted down in mapping. 

Both are provided for the following reasons:
First, mapping the samples differently provides a slightly different timbre. 
- maps will be slightly darker
+ maps will be slightly brighter.

Second, because these different Waveforms are specifically mapped so no samples overlap this allows you to layer a normally mapped wave and a shifted wave without any phasing. 

Finally, with waves that have been sampled with the natural (player’s) vibrato in them, you can layer two waves and get different rates of vibrato. 
- maps will have a slower vibrato than the normal map
+ maps will have faster vibrato than the normal map.