On the day that Manchester United unveiled their marquee signing of the summer, Robin van Persie, Sir Alex Ferguson also revealed that the Dutch forward may not be the final arrival at Old Trafford this summer.

Sir Alex Ferguson looked understandably delighted to have captured last season's Premier League top scorer on a four-year deal from rivals Arsenal.

"Robin is a world-class striker with a proven record in England and in European football," Ferguson said. "His talents need no introduction to our fans-he has scored a number of goals against us in some classic battles with his former club. His movement, finishing and all-round ability are outstanding."

Van Persie, 29, added that he had his heart set on a move to Old Trafford after deciding not to extend his deal with Arsenal.

"In these situations, when you have to make a hard decision in your life, I always listen to that little boy inside me" he said. "What does he want? That boy was screaming for Manchester United."

While the club has made a big financial commitment to Van Persie, Ferguson hinted that his transfer business may not yet be over this summer.

"I think that is maybe it [in terms of signings this summer]. Maybe in one other position," he told MUTV.

"We have spent a big amount of money and, if I need to go to the owners, I am sure they would say 'yes'. But it's only if a certain player would become available."

United have been linked with various players to bolster their midfield for some time and it could be that Ferguson looks to bring someone in before the close of the transfer window.

There has been further speculation surrounding United's interest in Benfica's Axel Witsel. Portuguese publication A Bola claims that United are now in pole position to sign the Belgium international as fellow-suitors Real Madrid close in on a deal for Tottenham's Luka Modric.

Witsel has been linked with United for several months following an impressive debut season in Portugal where he helped Benfica to the quarterfinals of the Champions League. He is likely to prove an expensive purchase, however, with the Lisbon giants claiming that they will only allow the 23-year-old to leave for his release clause of £31 million.

A more prudent option could be another midfield target, Hector Herrera. United have again been linked with a move for the 22-year-old who recently helped Mexico to a gold medal at the Olympics.

Vice-president of Herrera's club, Pachuca, has told ESPN Deportes that both United and Liverpool have approached the Mexican outfit about the player.

"We want him to continue in Pachuca," Andres Fassi said. "But he may leave the club at any time."

With Ferguson admitting earlier this summer to needing a left-back, it could be that any remaining funds are used in that direction. However, that could change if money were made available from a big-name departure.

There has been much speculation this summer about the future of Portuguese winger Nani, with the 25-year-old international yet to sign an extension to his deal at Old Trafford. The latest reports come from Russian agency, R-Sport, who claim that Zenit St Petersburg boss Luciano Spalletti is keen to bring Nani to the club.

Zenit have also been mentioned in connection with another United player, Dimitar Berbatov. Metro claims that the Russian club could be the club to finally bring the Bulgarian's time at Old Trafford to an end. The 31-year-old is thought to be available for £5 million and his exit is only likely to be hastened by the arrival of fellow-forward Van Persie.