Sforno on Deuteronomy 31:3/ספורנו על דברים ל״א:ג׳

1ה' אלוקיך הוא עובר לפניך, so that you have no reason to be saddened by my absence. You will enjoy leadership superior to mine, by G’d Himself orchestrating the crossing of the river. 'יהושע הוא עובר לפניך כאשר דבר ה. Although, strictly speaking, Joshua is not he people’s leader in that crossing, he will be the one who nominally will be leading you across the river, just as G’d has said.