Slice disks with only a minimum are bounded by the unknot

Question

Let $D\subset B^4$ be a properly embedded 2-disk in the 4-ball. Suppose the the radial function on $B^4$ has only a critical point on $D$. Then is $\partial D\subset \partial B^4=S^3$ the unknot? In other words, can the disk $D$ be isotopied into $\partial B^4$?