Petkovic announced in December that he would leave the capital club upon the expiry of his contract to take charge of Switzerland's national team.
However, Lazio argued the 50-year-old's negotiations with the Swiss federation violated his contract and, after much speculation, he was replaced at the helm by former coach Edy Reja on Saturday.
Petkovic's legal representative believes the club's reasoning was "unjust" and plans to take the case to the courts.
"We received notice of the sacking for just cause," Paco D'Onofrio is quoted as telling Sky Sport Italia. "We’ll impound the decision in the opportune legal arenas, because we consider it profoundly unjust.
"The statement released by Lazio is unique, as it doesn’t actually indicate the rule he is supposed to have broken during negotiations with the Swiss Federation.
"President of the Coaches’ Association Renzo Ulivieri also stated this week that Petkovic did not violate any regulation.
"No rule forbids a Coach from reaching an agreement with others when his contract is due to expire. Besides, Petkovic signed for a national team and not a club rival of Lazio."
