Ten Hag is set to be named new United manager next week.
McClaren had Ten Hag as his No2 when in charge of FC Twente.
He told The Athletic: "We were starting pre-season training the day after I was appointed.
"Ten Hag brought out this folder, and there were six weeks worth of pre-season training planned in there. Every minute of every day was laid out, from drinks sessions to warming down, to individual work.
"It was so detailed. I'd not seen anything like it before, and I've not seen anything like it since. He would prepare meticulous game plans about the opposition. How we were going to press, how we were going to build up.
"I thought I knew football before, but going there and experiencing that... I knew nothing about football."
He added: "He could see things in the game that nobody else could see, myself included.
"I have to sit up in the stands to see the patterns of a game, but he could stand on the touchline [and see them]. He always knew the answer to everything, how to change shape, change positions.
"His adjustments in the games — obviously I had worked with Sir Alex, and I used to think that some of his subs and changes were strange, but they always worked. Erik is the same in that respect."
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