McGrath wrote for the Sunday World: "Yes, Ole (Gunnar Solskjaer) could have started him, after all he is now heading off for international duty with Portugal.
"But you simply have to manage the man's workload cleverly.
"He's 36, not 26, a very fit 36 yes, and Cristiano has always looked after his fitness every well.
"But he still cannot go for 90 minutes in every game, not in English football, not if you want him to deliver in European action too.
"Did you notice yesterday how, a couple of times, after he came on, that Ronaldo was looking for fouls that the referee simply waved away?
"English football is more physical than Spanish football, and it is not as cynical as Italian football.
"But Ronaldo had to know that as soon as he showed an interest in joining any Premier League club that he was opening himself up to any number of defenders who would have no problem barging into him – and worse!
"It is brilliant for United to have Ronaldo back, but it is not all going to be plain sailing."
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