The Fed doesn't exit from QE and mop up excess reserves by selling long-term Treasury bonds for cash. The Fed exits from QE by raising the interest rate it pays on reserves so that they are no longer excess liquidity but rather investment vehicles.
So I do not understand Richard Koo at all when he writes:
Fed Will Need Treasury's Help Exiting QE: The next question is how to mop up the excess reserves that now amount to 17.8 times statutory reserves in the US. Given the size of the problem, I think any answer will have to involve the Treasury Department.A substantial portion of the funds supplied by the Fed were provided via the purchase of long-term Treasury securities. This means the securities the central bank has to sell as it winds down QE will consist largely of long-term bonds. But if the Fed begins selling long-term Treasurys at a time when the private sector has completed its balance sheet repairs and is starting to borrow money again, the result will almost certainly be a steep rise in interest rates that could stop the long-awaited economic recovery in its tracks.