Ferguson steered Posh to their third promotion in four seasons with a comprehensive victory at Old Trafford after late goals from Tommy Rowe, Mackail-Smith and skipper Grant McCann.
Yet Mackail-Smith, 27, looks set to leave London Road this summer after hitting his 35th goal of the season, with Barclays Premier League new boys QPR and Norwich and relegated West Ham all thought to be interested. Ferguson said: "You would have to say the odds are stacked with him probably leaving."
A switch to the big time would cap a remarkable rise for the highly-rated frontman, who made his international debut for Scotland in March, just four years after he was turning out in the Conference for Dagenham.
He added: "We would have to get the right money for him but everyone would wish him all the best. He's earned the chance to go and play at a higher level. We need other players and we have to try and establish ourselves in the Championship now."
Sir Alex Ferguson was not present to witness his son's victory but shortly after the game the London Road boss spoke to his father, whose Manchester United side were outclassed in losing 3-1 to Barcelona at Wembley in Saturday's Champions League final.
Peterborough boss Ferguson - a former player with United - said: "It's the best day of my career, both as a player and as a manager. With my dad losing, I wanted to get this win to put a smile back on his face. I've just spoken to him, and I've certainly done that, so I'm pleased and he's delighted.
"It was obviously a big weekend for the family and there were always going to be three different possibilities. The way it's turned out, at least one of us has got a win.".
It was the second successive season that Huddersfield had failed in the play-offs and another massive setback to the ambitious Yorkshire outfit. The Terriers had not lost in 27 previous league and play-off games, and boss Lee Clark admitted: "I'm hurting and I need to get away and spend some quality time with my wife and three children.
"I need to get my batteries recharged because I'm physically and mentally drained and then come back and see where we go. I know what it takes to bounce back because I've done it before. We've had an unbelievable season but we haven't achieved our goal."
Source: PA
Source: PA