The fear in Brussels and Washington is that Kiev will follow the failed course of a similar revolution in 2004-2005 and dissolve into political infighting between vested interests tied to powerful ministries and tycoons.
That era ended with the election in 2010 of a Kremlin-backed leadership that quickly realigned the former Soviet republic with Russia.
Scrutiny of Poroshenko is also growing because of the security service's attempt on Tuesday to arrest former Georgian president Mikheil Saakashvili -- an anti-corruption campaigner who is leading protests against the president.
The ruling party's desire to defang the National Anti-Corruption Bureau of Ukraine (NABU) by giving parliament the right to remove its leaders appeared to be the final straw for Kiev's chief Western backers.
IMF chief Christine Lagarde on Wednesday said she was "deeply concerned by recent events in Ukraine that could roll back progress that has been made in setting up independent institutions to tackle high-level corruption".
The US State Department and European Union issued similarly blunt statements this week. The World Bank and British Foreign Office have also rallied to NABU's defence.
The new agency has won numerous enemies by targeting people who seemed untouchable to law enforcement in the past.
These have included the powerful interior minister's son and three senior defence officials.
Poroshenko's party bowed to Western pressure by withdrawing the NABU bill from a debate planned for Thursday and agreeing to rewrite the legislation.
But analysts said they expected the attacks to continue.
"We have won this battle but the war goes on," Vitaliy Shabunin of Kiev's non-profit Anti-Corruption Action Centre told AFP.
'A disgrace'
The International Monetary Fund believes that corruption is a $1.8-billion (1.5-billion-euro) problem that accounts for two percent of Ukraine's gross domestic product.
But its true extent may be far greater because the handful of billionaires who control most of Ukraine's prized assets prefer to conduct their business through shell companies.
The extent of Western disenchantment with Poroshenko was encapsulated by Michael Carpenter -- a former US deputy assistant secretary of defense who is also a senior director of the Biden Center for Diplomacy and Global Engagement.
"If (parliament) votes to dismiss the head of the Anticorruption Committee (Yegor Sobolyev) and the head of the NABU, I will recommend cutting all US government assistance to #Ukraine, including security assistance," Carpenter tweeted.
"This is a disgrace."
Lawmakers agreed to remove Sobolyev hours after Carpenter posted his comments.
Conflict-riven Ukraine has relied on various sources of Western support to climb out of a dire 2014-2015 recession that nearly emptied the central bank.
Now those funds are drying up.
The European Commission last week decided against sending a 600-million-euro tranche payment of a 1.8-billion-euro assistance programme because of Ukraine's foot-dragging on institutional changes.
Poroshenko appeared to try Thursday to reassert his reformist credentials by vowing to introduce legislation supporting NABU's work with special anti-corruption courts.
But many remain unconvinced.
"The attacks against the anti-corruption agency will continue," Transparency International Ukraine chief Yaroslav Yurchyshyn told AFP.
"The question is whether the remaining clean members of the ruling elite can withstand pressure from the corrupt ones."
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