The Dane has been mentioned as a candidate for the United job after Monday's sacking of Erik ten Hag.
Ahead of their tie with Sheffield Wednesday tonight, Frank said: "Of course, I feel for Erik, he's a great coach and I think he's had a great career. I also think he's done a good job at Man United, winning two trophies in two years," said Frank.
"In a non-stop developing football world, I think he did, how can I say, more than acceptable. I know these big clubs, especially at Man United, there are massive expectations, but I also think there needs to be a little bit of a reality check because there are three clubs in a better place right now.
"City, Liverpool and Arsenal, I think that's fair to say, so when you're head coach/manager yourself, you always feel for the guys that lose their job, but I'm sure Erik put everything into it.
"I think there's better people than me to judge that, people who follow the club day in and day out. I know he had a very good first season."
Frank, on the chance of interviewing for the United job, continued: "You're right, it's a very delicate question and I have a big responsibility to Brentford football club and its fans, the leadership, staff, players and I have one focus.
"That's to do the very best against Sheffield Wednesday tomorrow in the Carabao Cup. I've said many times I'm happy here. I think it's one of the best clubs in the world. What happens in the future, who knows, but I'm very, very happy here."
Asked whether some jobs are too big to turn down, he added: "That's a good question but it's difficult for me to say. I always try to be as transparent as possible.
"I will repeat myself: I am very happy here and could see me being here for a long time. But I've also said if another opportunity comes up that maybe I wanted to try, let's see, but now, I'm very happy."