Cavani has been a big success since joining United in October, scoring 16 goals across 38 appearances, and earlier this month he signed a one-year contract extension to stay at Old Trafford.
But United boss Ole Gunnar Solskjaer has not ruled out signing another striker this summer as the Norwegian attempts to build on reaching Wednesday's Europa League final by closing the gap on Premier League champions Manchester City.
Former United striker Forlan played alongside Cavani for Uruguay and says he thrives on the competition that signing Tottenham's England captain Kane or Borussia Dortmund youngster Haaland would bring.
"Bringing in competition is good when you play so many games - and that is Manchester United every year," Forlan told the PA news agency ahead of the Red Devils' Europa League final date with Villarreal in Gdansk.
"Of course they can play alongside together like they play Luis Suarez and Cavani together in the national team.
"I don't think if one of them [Kane or Haaland] is going to come, if they have the opportunity to do it, then Cavani or the player who arrives to Old Trafford is going to be on the bench.
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"I think it's going to bring more competition and do good things as well for the young players.
Cavani turned 34 in February but Forlan, who won the World Cup Golden Ball for being named the best player at the 2010 tournament in South Africa, has not been surprised by his fellow Uruguayan's success in England.
"It's not only about the movement, which is really important in a football player because you don't play with the ball the whole 90 minutes," Forlan said.
"You have to time every chance because you touch the ball for a very few minutes.
"The way he plays without the ball is very important and also the way he scores goals."
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