The Red Devils have been long-term admirers of the 22-year-old although a broken foot curtailed a possible move in the January transfer window earlier this year.
Defour is now fully fit and performing at the level that caught the eye of some of Europe's biggest clubs, meaning the transfer speculation has intensified ahead of the window re-opening.
The Belgium international has confirmed his love for Standard, but thinks he will be forced to one day leave in order to further his career.
"I know that Manchester United are closely monitoring my situation. There hasn't been any official contact yet though," Defour told Extra Time.
"I've been sidelined for quite a while due to injury, but I'm playing at a high level again. The chance that I'll leave the club is rather big if I continue to play like I currently do.
"I've had the opportunity to leave Standard on more than one occasion in the past couple of seasons, but I loved the club too much to leave. However, I'm 22 years old now and I also have to think about myself.
"There comes a time when you just have to make the next step in your career, regardless of sentiments for any club."