He told the Daily Star Sunday: "When Sir Alex plays Hernandez, Rooney and Van Persie all together, that really is a frightening trio.
"One that rivals the Rooney-Ronaldo-Tevez trinity which helped United lift the Champions League in 2008.
"Add in Danny Welbeck as well - an England international who is fourth-choice striker at the club - and you have an enviable set of attacking options.
"I managed teams against that Rooney-Ronaldo-Tevez combination and it was tough going. You could keep one, maybe two, quiet but you ran out of quality players to mark them.
"You didn't know which one would provide the threat on any given day. But you were certain at least one would.
"And if it was a day when all three were on song - well you were in trouble. Hernandez looks full of confidence again when he plays, like the player who first broke into the United side two years ago.
"And he is scoring goals for fun. One of the problems when you are at a club like United is that when the quality runs so deep, there are always going to be world-class players sitting on the bench.
"Players who would find themselves walking straight into another team find themselves not playing for weeks on end.
"That's how and why United stay at the top. That competition for places and hunger which exists between the players.
"Wayne and Robin will recognise that Hernandez is a serious threat to them. He's not just some kid - he is a lad who is a real proven goalscorer who is now showing the ability he has when given the chance. And when they are all in there together, well as I have said, it's frightening."