The Portuguese coach, who lived in Spain whilst in charge of La Liga side Real Madrid from 2010 to 2013, is said to have defrauded the Spanish government of £3m, something he staunchly refutes.
However, allegations of tax fraud - denied by the forward - have brought Ronaldo's future into question and United have been heavily linked with a swoop for their former star.
The Real Madrid star was unmarked at the back post, where he rose to head Raphael Guerreiro's cross past Russian Federation keeper Igor Akinfeev and into the net after just eight minutes.
Cristiano Ronaldo remained silent on his Real Madrid future after taking no questions during a postmatch news conference following his Man of the Match award in the wake of Portugal's 1-0 Confederations Cup victory against Russian Federation in Moscow.
It was recently reported that the 32-year-old is fed up with life in Spain and now wants to leave Real Madrid in the transfer window.
In an earlier report Marca said Ronaldo has told his teammates he was leaving and "there is no turning back".More news: Erdogan rejects Arab demands; Turkish troops stay in Qatar
He also confirmed that Ronaldo s contract was ring-fenced by a one billion euro release clause.
"But something really weird would have to happen if he were to leave this club".
The win moved Portugal on to four points - they drew their opener 2-2 with Mexico on Sunday - and avoiding defeat against unfancied New Zealand on Saturday in St Petersburg will be enough to see them into the semi-finals. The prosecutor said Ronaldo used shell company in the Virgin Islands to cheat on his taxes.
Ronaldo claimed the club had not protected him sufficiently after he was charged with allegedly evading £13million in tax.
His first big challenge is keeping Ronaldo at the club he joined in 2009 from Manchester United for a record 94 million euros ($105.3 million).
Despite interest, Ronaldo is not, however, a priority deal for United.