Fed officials decided just this year to stick with that new operating approach, which had proven reliable while allowing the central bank to be less active in markets.

To stick with the new approach, the Fed had to commit to keeping its balance sheet bigger than it was before the last recession — back then, reserves were scarce, and they now need to be ample. To that end, the Fed stopped its effort to shrink its balance sheet as of last month.

But the fact that intervention was needed suggests that it may have allowed its balance sheet to get too small for the new approach to work without active help from the Fed when market conditions are irregular.

Even though the overall amount of reserves in the banking system remain high, with excess reserves at about $1.4 trillion, cash no longer flows out readily to smooth over temporary shortages. The hoarding happens partly because banks now hang onto their reserves to satisfy post-crisis rules .

That enables market stress like this week’s gyrations, and means that the Fed can set things right with a relatively small intervention. The Fed’s operation to inject funds ultimately amounted to just over $53 billion.

“Without flexibility in balance sheets, these types of spikes in repo rates may be unavoidable,” said Seth Carpenter, United States chief economist at UBS and a former Fed official. “The Fed may need to be ready.”

The central bank could lower the rate of interest it pays on excess reserves at its meeting Wednesday, a technical tweak that it has made several times to help to keep rates within its desired range. The Fed is expected to lower its benchmark interest rate at that meeting by a quarter-point.