Ighalo, who spent six months at Old Trafford in 2020, finished as top scorer in the Saudi Pro League last season for current side Al Hilal.
But the Nigerian's exploits were upstaged by the arrival of Ronaldo who secured a money-spinning switch to Al Nassr.
Ighalo spoke about his own motivations for playing at the age of 34 before claiming the Portuguese star only signed for Al Nassr for money.
"When you are young, yes, you play for passion. Back then, you don't care about money," He told OmaSportsTV.
"But at my age, I am in the final stretch of my career, I don't know if it will be one, two years or when God will tell it to stop.
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"I know it won't be more than three years.
"I've played for passion all my life, now it's for money.
"I'm not one of those players who come and say: 'I play for passion'. Brother, it's money. At the end of the day, it's money."
He continued: "Is Ronaldo still playing out of passion? Ronaldo has earned 100 times more than I have in my entire life and yet he went to Saudi Arabia.
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"Did he do that out of passion? It's for money, bro."
Ronaldo netted 14 goals in 19 games since arriving in the Middle East but was unable tio help Al Nassr claim the Saudi title.
His arrival has opened the door to a host of stars that will grace the Saudi Pro league next season including Karim Benzema, N'Golo Kante and Jordan Henderson.
Ighalo is a free agent after leaving Al Hilal at the end of last season. He moved to Saudi Arabia following his short stint at Old Trafford.
A season with Al Shabab where he scored 21 goals preceded last season's exploits at Al Hilal.