The Manchester United boss also hailed the work of Pep Guardiola at Barcelona, but insists the Spaniard will not be returning to management soon as he continues his sabbatical
The Barcelona forward has capped an impressive personal campaign by surpassing Gerd Muller's 40-year-old landmark of 85 goals in 1972 and currently stands on 90 strikes in 2012 with one La Liga match remaining.
Ronaldo, meanwhile, ended on 46 goals in the Spanish top flight last term, four behind Messi, but won his first La Liga with Real Madrid since his arrival in July 2009.
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"I think that what we are seeing at the moment from Messi is an exceptional player from this decade along with Cristiano Ronaldo.
"There is nothing between the two of them, they are both the best players in the world."
In September, Goal.com revealed that Sir Alex has targeted Pep Guardiola and Jose Mourinho as the only two candidates capable of succeeding him at Old Trafford and the Manchester United manager hailed the Spaniard's work at Barcelona.
"His [Guardiola] record is fantastic; the trophies he won at Barcelona were quite exceptional and that is his CV, he wins trophies," Sir Alex continued.
"Of course but Barcelona have always had that style and played attractive football. He increased that by putting a new work ethic into the players in terms of their work rate. They work extremely hard to get the ball back and if you combine that with their possession it makes a fantastic team.
"I spoke to Pep a little while ago and he said that he has no interest in getting back into management at the moment."
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