The Portuguese has made clear his desire to leave the club amid a disastrous summer on and off the pitch, culminating in Saturday's 4-0 defeat at Brentford.
Ronaldo's behaviour in that game formed part of the discussions the United staff had in the aftermath, as they attempt to work out how to navigate the team back to good form.
There is increasing debate within the club over whether it would be better to get rid of the megastar, although the hierarchy have so far been completely resistant.
Joel Glazer is said to have been a particular backer of the Portuguese, both due to his goal return and his commercial value.
Sporting have approached United about a deal, though. Ronaldo's agent Jorge Mendes has spent the summer investigating potential options, but none of the super clubs want the 37-year-old, and few below that would be able to afford a fee or his wages.
Sporting can make a deal work if they can sign Ronaldo on a free. Atletico Madrid, who also have a strong relationship with Mendes, have been considering a similar move but are still mindful of backlash for their fans.
Any deal with the Spanish club could at least aid United's long-standing interest in Joao Felix, who is commonly seen as Ronaldo's successor as Portugal's next megastar.