Since Cristiano Ronaldo just renewed his contract with Al-Nassr a few days back, I agree a deal that will make Cristiano Ronaldo move to Manchester United from Al-Nassr won't ever happen anymore because by the time Cristiano Ronaldo's contract with Al-Nassr expires, Cristiano Ronaldo will be 42, which is not applicable for Manchester United to sign Cristiano Ronaldo at that age. Cristiano Ronaldo is likely to move to Sporting CP after his contract with Al-Nassr expires because he made it clear he will retire at Sporting CP.
Not even for the sake of retirement will Ronaldo want to go back to Manchester United and this has got nothing to do with hate as some persons may want to make it seem as. As at now, playing in England does not seem like a dream Ronaldo would want to come through again and just like you said he has mentioned about retiring at sporting but I'm even thinking it will be possible he even end up retiring at Al Nasrr seeing that by the expiration of his new contract, he will be due for retirement, sporting may not want to engage him except he will be going as a free agent and since it will be for the purpose of retirement, he may not even really have much play time and same may apply to his wages and other incentive but on a second taught, it's going to be a honour to sporting that Ronaldo gets to retire at that team which they may not want to turn down knowing the honor that will bring to them.