In days running up and after the elections on July 9, Indonesia saw a flood of capital inflows.
The rupiah gained 4.88 percent to touch 11,495 against the U.S. dollar in the 20 days between June 25 and July 23. Since then, the rupiah has trended weaker as the excitement over Widodo's election faded, said Goh.
But with the current account deficit running at 3 percent of gross domestic product, "the inflows weren't sustainable," he said.
Goh has a target of 13,250 rupiah against the U.S. dollar, originally for the end of 2015.
Not only do investors have reason to sell the rupiah, but the dollar's strengthening momentum remains sold, he said. The dollar index has been perched at 12-year peak since last year.
"We could get there earlier of if the U.S. dollar continues to strengthen – particularly if next week's non-farm payrolls are strong," he said.
But a fall to the 13,200 rupiah level "could prompt foreign investors to start hedging their local currency assets and provoke a vicious cycle of capital outflows", warned Macquarie's Idris.