The 37-year-old made a shock return to Old Trafford last summer, joining from Juventus for a deal rising to 23 million euros (£19.6m).
Ronaldo scored 24 goals in all competitions and was named Sir Matt Busby Player of the Year, but individual quality was unable to prevent his first season back at United ending trophyless and sixth in the Premier League.
Ronaldo has reportedly asked to be allowed to leave if a satisfactory offer arrives, but a senior United source indicated to the PA news agency that the forward is not for sale.
The stance was the same last weekend when it emerged that agent Jorge Mendes had discussed his client in a meeting with new Chelsea owner Todd Boehly.
Ronaldo has one year remaining on the deal he signed last summer and new United boss Erik ten Hag has expressed his excitement at working with the record-breaking Portugal international.
United’s international players are due to report for training in the coming days ahead of flying to Thailand on Friday for the start of their pre-season tour.