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Forlan believes that Van Gaal still needs a number nine to back up Wayne RooneyGetty

Former Manchester United striker Diego Forlan has urged Louis van Gaal to sign a number nine – apart from Pedro Rodriguez – before the end of the transfer window, as Wayne Rooney cannot cope alone with a busy season where the Premier League giants will return to the Champions League. The Dutch boss has bolstered all the lines of his squad during the summer transfer window after spending around £80m ($125m) in signing promising goalkeeper Sergio Romero, winger Memphis Depay, right-back Matteo Darmian and midfield duo Bastian Schweinsteiger and Morgan Schneiderlin.

Van Gaal, however, is yet to sign a number nine despite having allowed both Robin van Persie and Radamel Falcao to leave the club following disappointing seasons. Rooney is expected to return to the attack in the new campaign after the captain played as a midfielder during much of Van Gaal's first year at Old Trafford.

But Forlan believes that even though the club captain is more than capable of leading the attack, he still needs a back-up – especially considering that with Javier Hernandez's future at the club up in the air, academy star James Wilson is the only other striker in the squad.

"Wayne Rooney is good enough to play in almost any team in the world: he's experienced, the club captain, who I'm told trains harder than any other player, but my old club need more than one natural goalscorer if they want to win a trophy in 2015/16", Forlan wrote in his column for UAE newspaper The National.

Manchester United have been heavily linked with a move for Barcelona winger Pedro Rodriguez, as van Gaal himself has insisted the club need speedy attackers to fill the gap left by Angel Di Maria, who has left for Paris Saint-Germain. The Uruguay former striker backs the potential arrival of Pedro, but insists the Dutch boss still needs to address the departures of van Persie and Falcao with a new number nine.

"They have very good creative players such as Juan Mata, Memphis Depay, Ander Herrera and Ashley Young. They look like they're going to sign Pedro, from Barcelona, and that will be a wise addition. But while the creative players will chip in with their share of goals, they're not the natural-born finishers who live and die by how many goals they score, the predators who play on instinct and just know where the goal is," he added.

"United have lost Robin van Persie and Radamel Falcao, two great strikers who struggled last season. It was the club's decision to let them go, but I'm surprised that they've not been replaced."

Forlan, meanwhile, has claimed that Pedro's move to Old Trafford from Barcelona also makes sense for the player, comparing the Spaniard current struggling situation at the Camp Nou with the one he experienced during his time at Manchester United.

"Pedro would be a versatile signing who can play in four or five positions, including the central striking role. I understand why United would be an attractive move to the Barcelona forward: United are another huge club who should play — and pay — well. The 28-year-old Spain international should also find it easier to get into a side in transition than a team that already has Lionel Messi, Luis Suarez and Neymar up front," Forlan wrote.

"He'll move for more game time, which is exactly why I left United, a decision I never regretted. United had Ruud van Nistelrooy, Louis Saha, Solskjaer, Alan Smith and Cristiano Ronaldo when I departed."