In its comment on John 14:28, the second edition of the The Ignatius Catholic Study Bible reads:
The Ignatius Catholic Study Bible - John 14:28
the Father is greater: The Son is equal to the Father in his divinity but less than the Father in his humanity. Although no one of the Divine Persons exceeds the others in greatness or glory in the eternal Trinity, there is a relational hierarchy among them, where, unlike the Son and the Spirit, the Father alone possesses divine Paternity and has the distinction of being entirely without origin.
Above, the Ignatius Commentary explains Jesus's statement, "my Father is greater than I", by saying that Jesus was only speaking with respect to His humanity. A form of this explanation is used in Trinitarian apologetics to explain nearly every text which appears to teach that Jesus is subordinate to and less than the Father, although neither Jesus nor the Biblical authors ever make such a qualification.
Under such a reading, Jesus would equally be saying, "My humanity is less than my own divinity", as classically speaking, He is consubstantial with the Father, and is the same Being. And, from a Trinitarian perspective, if He were speaking with respect to His divine nature, He could say, "The Father is not greater than I", or, "The Father is equal to me", also without providing any warning or context to explain that He only means what He says with respect to part of Himself, and that for another part of Himself, which He never mentions, the statement would be untrue.
However, notice that the Ignatius Commentary goes on to state that "there is a relational hierarchy among" the members of the Trinity, where "the Father alone possesses divine Paternity and has the distinction of being entirely without origin". This makes it seem as though they are suggesting that one is to understand Jesus saying "my Father is greater than I" with respect to His divine nature as well, in some sense that in their view technically preserves their equality.
It is true that under the classical understanding of the Trinity, only the Father exists of Himself, with the other two Persons of the Trinity having Him as their "origin", and therefore lacking aseity (self-existence). This creates the difficult situation where one has to affirm that the Son and Spirit are fully divine, and equal to the Father, yet they have Him as their origin. It is hard to see how supposedly eternal Persons within God can have a source or origin in another Person. And, it would suggest that the two Persons who do not exist of themselves would be lesser than the Person who has aseity. If the Ignatius Commentary is suggesting that Jesus said "my Father is greater than I" in reference to the fact that the Father has aseity and He does not, then that would seem to establish a clear difference between them, and a legitimate inferiority on the part of Jesus with respect to the Father.
In any case, John 14:28 can be taken as a relatively straightforward statement by the Lord establishing that He is under the one God, and that the one God is greater than Him, as is taught elsewhere in the New Testament as well (Matthew 12:17-18: 1 Corinthians 11:3, 15:27-28, etc.).