If losing Ronaldo, who earns around £100,000-a-week, is the only way to get to Beckham, it is understood that Inter chairman Massimo Moratti would be prepared to consider the swap.
Ronaldo is also the biggest soccer client of Nike, who will take over from Umbro as United's k i t suppliers next season.
Kenyon fears that Beckham will come back from this summer's World Cup Finals with only a year left on his contract and six months away from a Bosman free transfer.
United are also keen to tie captain Roy Keane to a new deal but Kenyon said: 'That is not something we have started to progress yet.'
Louis van Gaal, United's preferred choice to take over as manager from Sir Alex Ferguson in the summer, confirmed his interest in the job.
'There have not been any negotiations so far but I have heard that there is interest in me from United,' said the former Ajax and Barcelona manager, who finally stood aside as Holland coach yesterday.
'They are one of the top names in Europe and it would be a very appealing proposition.
'Any manager with ambition and belief in himself would love to work for a club as big as United and I certainly believe I belong in that category.'