The 22-year-old moved to the Estadio Jose Alvalade from Utrecht in the last transfer window, but already is being linked with a move to a bigger club, with the Premier League champions one of his potential suitors.
The Dutch striker has made an electric start to his time with the Portuguese club, scoring six goals in his first six games, which has come to the attention of a number of leading European teams.
Despite remaining coy about a move for the forward, Phelan has admitted United are keeping tabs on the Netherlands international.
“We have obviously known Van Wolfswinkel for a long time. He is a young striker, very talented, who has been growing in the last few seasons,” Phelan stated to Portuguese newspaper Record.
“A club like Manchester United carefully follow many players all over the world.”
Sir Alex Ferguson has signed players from the Portuguese Primeria Liga before, with the likes of Cristiano Ronaldo, Nani and Anderson ending up at Old Trafford after impressing in the country.
By Gareth McKnight