Diaper was at Wembley to watch McMenemy’s Saints famously upset the odds to beat Manchester United and win the FA Cup in 1976.
The former Dell boss surprised Diaper with a phone call out of the blue on Saturday morning to wish him luck and give him some words of advice.
Diaper said: “In ’76 I was here. I won’t say how old I was but I remember it.
“What made my day was I had a phone call this morning from Lawrie McMenemy.
“It was tremendous.
“I am a great Saints fan and he said ‘it’s important to tell your players there is all the hype and publicity but it does come down to 90 minutes of football.’ “That’s basically what I said to the lads before the game.
“I really appreciated that phone call. It made my day.
“Some of my players have probably never even heard of Lawrie McMenemy but he was great for Southampton Football Club and it meant a great deal to me.”