“Manchester United has this morning initiated appropriate steps in response to Cristiano Ronaldo's recent media interview,” a club statement released on Friday said.
“We will not be making further comment until this process reaches its conclusion.”
The second part of the interview aired on TalkTV on Thursday. Ronaldo used his conversation with Piers Morgan to say he does not respect United manager Erik ten Hag and to claim that senior figures at the club are trying to force him out of Old Trafford.
Talks have begun between United and Ronaldo's representatives over the Portuguese international's future at Old Trafford, where he is under contract until next summer, the Press Association news agency reported.
A potential exit has been complicated by the limited number of clubs being able to meet his reported salary of about £500,000 ($590,000) per week.
If he is found to be in breach of contract, a possible outcome could be the cancellation of his deal.
United officials were not aware any interview had taken place and had to wait until it was broadcast before determining the club’s legal position.
The club said it wants the process to be concluded swiftly.
Ronaldo’s comments are the latest in an acrimonious few months since ten Hag took charge at the end of last season.
When he refused to come on as substitute against Tottenham, United’s manager dropped him from his squad and forced him to train away from the team.
“I think he don’t respect [me] the way I should deserve,” Ronaldo said. “But it is what it is. That is probably why the game against Tottenham, I left … This is why I say I don’t have respect for him, because he don’t show respect for me.
“This is why we are in that situation. I have to be honest that things are not going well because of that. Because the empathy don’t exist.”
Ronaldo, who failed to secure a move to a Champions League club in the offseason, paved the way for a potential departure in the January transfer window.
“Maybe it’s good for Manchester and probably is good for me as well to have a new chapter,” he said. “Probably. But I don’t know.
“If I will be back, I will be the same Cristiano. But I hope people will be on my side and let me shine like I did at all the clubs and all years.”
United could move to sack Ronaldo and start a potentially long, ugly legal battle. Another option would be paying him to go, which, given his exorbitant wages and desire to leave, looks unlikely.
Finding middle ground is a third option that may suit both parties, as he has made it clear he wants to leave and the club are ready to cut ties.