"It’s unlikely that growth is going to be more than 4 percent this year," he told reporters Friday after holding policy rates.
The economy was expected to grow by about 3.4 percent in the first half of 2018, and will have to grow by 4.6 percent during the latter half of the year to reach 4.0 percent.
A recovery in the agriculture sector and related industries will help growth.
Coomaraswamy said there was a negative output gap, and real interest rates were also high, with inflation in mid single digits indicating space for a policy rate cut, but major central banks were raising rates and Sri Lanka may face capital outflows, prompting the monetary authority to hold rates.
The central bank triggered a minor run on the rupee by printing large volumes of money in March and April to enforce a rate cut, but conditions are now stabilizing, after short term rates rose.
Analysts have pointed out that the biggest problem in Sri Lanka is the tendency of the central bank's domestic operations department to inject cash into the banking system though sudden 'quantity easing' style exercises, which tends to de-stabilize the credit system and the currency peg.
Services and industrial sectors are also expected to expand, the Central Bank said in its monetary policy statement Firday.
"The economy is projected to reach its potential over the medium term benefiting from a competitive flexible exchange rate, a low inflation environment and a stronger policy framework to support exports and investment, amidst continued fiscal consolidation."
Last year growth fell to 3.2 percent, amid a credit slowdown and foreign reserve collections.
Coomaraswamy said there was a 'squeeze' on the economy from both monetary and fiscal tightening.
Sri Lanka is also expected to miss an International Monetary Fund reserve target for mid 2018, but will apply for a waiver.
The Central Bank collected over billion dollars of reserves in 2017, a practice which tends to dampen credit and growth, in general.
Analysts have pointed out that there is a credit recovery 18 months after the peak of the most recent balance of payments crisis. In the 2011/2012 crisis it took 26 months for credit to recover.
However this year there is an added dampener of a permanently depreciating currency which tends to kill domestic purchasing power and also generate political unrest.
Earlier this year, the Central Bank had expected growth to be around 4-4.5 percent. (COLOMBO, 3 August, 2018)