問題は誰が彼のかわりにそこへ行くかだ。

。

The question is who will go there in his place.

Literal

Problem [topic-は] who [subj-が] his place [in-かわりに] there [to-へ] go [embedded-Q-か] [copula-だ].

~のかわりに ('in place of, instead of') is the compound expression for substitution. Another embedded wh-question — 誰が...か ('who will X'). The pattern 問題は[wh-clause]か+copula is a frequent formal discussion opener.