Last Updated: Wed, 27 Mar 2013 10:24
United watched Argentina defender Ezequiel Garay again while AC Milan have also shown their interest in Porto's Eliaquim Mangala.
Sir Alex Ferguson had a spy at the Monumental Stadium in Buenos Aires on Friday to see Garay, 26, help his country keep a clean sheet in a 3-0 World Cup qualifying win that maintained their spot at the top of the South American table.
The rumours in Portugal point to a potential £17million summer switch to Old Trafford, but United are also interested in Porto's French centre back Mangala.
Ferguson's brother has also been keeping a close check on the 22-year-old ex-Standard Liege defender who has been arguably in better form than Garay recently.
Benfica and Porto are battling it out for the Portuguese title at the moment but they are also rivals in the transfer market where their financial model is based on selling on one or two star players every summer.
Ferguson will have a preference for which of the two targets he would prefer to add to his squad - but headlines concerning both may serve to keep the asking prices down.
AC Milan are in the market for a new centre back this summer and have also been linked to Mangala, while Barcelona appear to be the Reds' most serious current rivals for Garay - not Manchester City.
Have you an opinion on this story? Put your money where your mouth is and claim a free £25 bet.