Because the man that Utd wanted, was available now, and might not have been at the end of the season?
This suggests it was the other way around, that he wanted to stay in Portugal until the end of the season but the Utd board told him it's now or never?
https://www.bbc.co.uk/sport/football/videos/c4gx2xxkd7xoIf that is true then RVN having a caretaker role until the end of the season was never on the cards?