If you are working in LR, select the Adjustment Brush (K key) and enter the value for reduced saturation (–10 or –20), and, perhaps, reduced exposure and sharpness while you're at it, then "paint" over the background wherever you want the changes to be made. You turn on (and off) the mask so you can see where you've painted using the "O" key. You can also adjust the feathering of the brush to get just the right edge.
Once you've "painted", turn off the mask and have a look. You van increase or decrease any of the adjustment brush settings while looking at the image to get just right balance.
Adjustments brushes are what make LR so incredibly versatile as you can also go back at any time and edit them further!