Mikel joined Lyn Oslo when he was under 18
Morgan Andersen, former director of Norwegian club Lyn Oslo, has been handed a one-year suspended jail sentence by an Oslo court.
He was found guilty of forging contracts relating to Chelsea and Nigeria player John Mikel Obi in 2005.
The court said the falsified documents linking Lyn and Mikel effectively enabled the club to gain a slice of the player's transfer fee.
Andersen told the court he would lodge an appeal against Wednesday's sentence.
Mikel went on to become embroiled in a transfer row between English Premier League clubs Manchester United and Chelsea.
"The court is in no doubt the contracts were amended and Andersen knew this," judge Torjus Gard said in his judgement.
Mikel was under 18 when he joined Lyn and was therefore given an amateur contract.
After the midfielder turned 18, the Norwegian soccer authorities were presented with a professional contract Mikel has denied signing.
The documents with Mikel's signature on were cut out from the amateur contract, the judge said.
Mikel joined Chelsea in June 2006 after suddenly announcing he would not go to Manchester United from Lyn as expected.
Mikel said he had been put under pressure to sign for United.
Chelsea said they had reached a financial settlement with United and Lyn under which they would pay the Old Trafford club $24m and Lyn $8m.
Andersen is now on leave from his job as director of another Norwegian top flight club, Fredrikstad.
His lawyer Cato Schioetz told Norwegian broadcaster NRK he would probably not be able to return to that position after the court ruling.