Seems to me the only way to get it back in place is to remove the lower fuel tank.
You can easily get it back into place under the lower fuel tank by using a water-based lubricant such as KY jelly, which is available at any drugstore. Flood the gap between the lower fuel tank and the frame with water from a hose to get rid of any debris that may be there, apply the KY lubricant to both sides of the aft end of the rubber mat, and it will slide right in. Then hose the area down again to remove the lubricant.
The benefit of using KY, rather than any other lubricant, is that it is water-soluble and will not damage the mat or any other components it comes in contact with.
The mat is very important, it prevents heat from the engine from boiling the fuel in the throttle body when you are stuck in heavy traffic in the summer. It also reduces heat transfer upwards to the upper fuel tank and the rider's crotch.
Michael