Eriksen impressed in victory over Burnley on Saturday.
Frank said of his fellow Dane: "It's never been a gamble for me. I have been convinced that he could help us and if we could get him, it would be a great transfer. Maybe even the best in Brentford's history.
"Or maybe the most important - depending on how you look at it. He has not been injured as such.
"He had heart problems, but that has not made him lose speed or anything. So it's all been about whether he would be mentally ready."