"This chapter is over," Ronaldo cryptically said on social media with a photo of him in an Al-Nassr shirt.
Ronaldo, the top scorer in this season's Saudi Pro League, has eight in seven appearances in this campaign's Champions League Elite, one short of joint-top scorers Riyad Mahrez, Salem Al-Dawsari, Anderson and Jasir Asani.
The 19-year-old forward, who trains daily alongside one of football’s all-time greats, is part of a promising Saudi Arabian generation aiming to make a statement on the global stage.
Al-Nassr remain in the third place with 54 points but they are now three points behind second-placed Al-Hilal and seven off leaders Al-Ittihad, who visit Al-Ahli on Saturday.
Cristiano Ronaldo's Saudi side Al Nassr were on Monday drawn against last season's runners-up Yokohama F-Marinos of Japan in the quarter-finals of the Asian Champions League.
Ronaldo, the Al Nassr captain, was reportedly rested for last week's goalless draw in the first leg in Iran, however returned to lead the side at Al Awwal Park.
Ronaldo turned 40 last week with his current contract due to end in June 2025.
Al Nassr captain Ronaldo had looked destined to endure a hugely frustrating game following a series of missed opportunities in the first half at Al Bayt Stadium.
The Portuguese star, without a goal in his past three appearances for the Saudi Arabian side, netted the second of the hosts' goals at Al Awwal Park in Riyadh.
Ronaldo stepped up to take the penalty and the 39-year-old sent the goalkeeper the wrong way to score his sixth goal of the league campaign in the seventh minute of stoppage time.
Cristiano Ronaldo insists he is focused on being the perfect team member at Saudi side Al Nassr rather than collecting more individual accolades in the twilight of his career.
Ronaldo, making his first appearance in the rebranded Asian Champions League having missed last week's 1-1 opening draw against Iraqi club Al Shorta, had a goal disallowed for a narrow offside at the beginning of the second half.
His Portuguese predecessor Castro was the third coach to depart Al Nassr since Ronaldo's groundbreaking arrival in early 2023.
Al Nassr captain Ronaldo did not travel with the squad for the match in Baghdad, with the club issuing a statement a day before stating that the Portuguese superstar had been diagnosed with a viral infection.
The Portuguese veteran is targeting Asia's top club crown to add to his long list of achievements when the competition kicks off on Monday.
Honouring Cristiano Ronaldo's achievement of being the first and only player to reach the incredible milestone of 900 career goals, Al Nassr presented the Portuguese forward with a special jersey ahead of their Saudi Pro League game against Al Ahli on Friday.
Ronaldo's latest goal places him just two goals shy of achieving 900 career goals.
Saudi Pro League champions Al-Hilal battled past Cristiano Ronaldo's Al-Nassr 5-4 in a penalty shootout on Friday to win the Saudi Cup and claim the domestic double, leaving the Portuguese forward in tears at the end of the drama.
The Portuguese 39-year-old scored a brace in his side's 4-2 win over Al Ittihad on the final matchday to bring his league tally this season to 35 goals.
