I am not entirely sure what you mean (I'll elaborate below), but the Eucharist will always be the source and summit of the Christian life and the focus of the Church's liturgy. The Eucharist is the participation in the saving Sacrifice of Christ on the Cross which delivered us from our sins. It is how we apply His Blood to our sins and our lives today, and therefore it is of supreme importance.
Moreover, it is God Himself, and through receiving Him we become partakers of the divine nature (2 Peter 1:3-4) and are filled with all the fullness of God (Ephesians 3:19).
St. Ignatius of Antioch, martyred in A.D. 107, called it
"the medicine of immortality, the antidote to death".
It is the new manna come down from Heaven (cf. John 6:25-33). It effects union with God; that is why it is called Communion. Through it the Church, the Bride of Christ, becomes one Flesh, one Blood with her Bridegroom. In it, we now have access to the fruit of the Tree of Life. It is the bread of angels, the cup of salvation prophesied by the Psalmist (116:13). Through it we abide in Christ and as branches draw sap from the vine. It is the banquet of the wedding feast; in it we taste and see that the Lord is good (Psalm 34:8). In it we say, "The Lord is my portion and cup; it is you yourself that I claim for my prize" (Psalm 16:5).
Through the Eucharist (the word from the Greek for thanksgiving), we enter into the One Sacrifice of Christ on the Cross and partake of the Flesh of the Paschal Lamb sacrificed for our sins. Christ spoke of a New Covenant in His Blood.
In the Biblical perspective, a covenant relationship is sealed with a ritual — often involving blood. Christ died once for all on the Cross to free us from sin as the Lamb of God, the Passover Sacrifice that delivered us from bondage to the slavery of sin. When God led the Jews out of slavery in Egypt, He first had them celebrate the Passover: The lamb was sacrificed, its blood smeared on the doorway, and its flesh eaten in the Passover ritual. Examine the Passover ritual in Exodus closely. Without doing all three of these things, the Angel of Death would strike down their firstborn. The Paschal feast also fed the Jews through their journey in the desert, as did the Manna, into the Promised Land of milk and honey.
Jesus's Death on the Cross fulfills the sacrifice. Christ, as Scripture says, is our Passover (1 Corinthians 5:7): the Lamb of God, who was sacrificed for our sins, of whom the Exodus lamb was a type. But unless each one of us partakes of the flesh of the sacrificed lamb, as the Israelites did, we do not truly participate in the sacrifice. For what Jew would merely say,
"I claim the blood of the Paschal lamb over my sins."
- and leave it at that, trusting that that was enough?
No, he would also eat the flesh of the sacrifice.
So, too, is the Eucharist the sign of the New Covenant with God — it is the One Sacrifice of the Lamb of God on Calvary made present for each of us at the Eucharist, the flesh of the Lamb of God sacrificed for us to partake, and the blood of the Lamb to smear on our doorposts (symbolized by our lips). The Last Supper was a Passover seder meal.
But if Jesus is the Lamb of God who was slain, there remains, according to the Passover type, one more step: not only was the lamb sacrificed, but its flesh was eaten. And the flesh had to be eaten: it was eaten during the Seder meal. Through the Seder meal, the Jews received the lamb's sacrifice: it was not enough for them to merely say,
"I claim the blood of the lamb over my sins" but neglect to eat the sacrifice.
So, too, in order to enter into the sacrifice of the Lamb of God on the Cross, we must eat the flesh of that sacrifice. Through the most wonderful gift that Jesus left us for his remembrance, we are able to enter into the One Sacrifice of Christ on the Cross and truly receive the sacrificed Flesh of the Lamb of God so that we, like the Jews of Exodus, may eat it. This sacrifice of the New Covenant is the pure offering prophesied by Malachi:
11 My name will be great among the Gentiles, from the rising to the setting of the sun. In every place incense and pure offerings will be brought to my name, because my name will be great among the Gentiles.
The word for offering in this verse, <mincha>, means a food or grain offering. When it occurs in Scripture, it almost always in the context of cultic sacrifice, and then, almost always in the context of a grain offering.
This is our Eucharist: an Entering-in or Re-presentation of the One Sacrifice of Christ. As such, the Eucharist is a sacrifice, or rather is THE Sacrifice: it is one and the same as Christ's Sacrifice on the Cross, with the same priest (Christ) and the same victim (Christ). This Sacrifice feeds us so that we may escape slavery to sin (as symbolized by the slavery in Egypt) and travel through the spiritual desert of this life (the forty years), fed continuously by Manna from Heaven (again, a type of the Eucharist), until we reach the Promised Land.
St. Paul draws a critical parallel between the Passover Seder and the Eucharist. He says,
|7 Christ our Passover was sacrificed for us. 8 Therefore let us keep the feast.
(1 Corinthians 5:7-8)
Not the Jewish Seder, for the Old Law had passed away.
Paul is referring to the Eucharist, which he calls a participation in the body and blood of Christ, that is, the means by which we enter into The One Sacrifice of Calvary.
16 Is not the cup of thanksgiving for which we give thanks a participation in the blood of Christ? And is not the bread that we break a participation in the body of Christ?
(1 Corinthians 10:16)
This is the cup prophesied by the Psalmist when he said,
|13 I will take the cup of salvation, and call upon the name of the Lord. (Psalm 116:13).
This was the very same Psalm used in the Seder — called the Hallel. Jesus and the Apostles would have sung it just before they got up to go to Gethsemane, where Jesus contemplated the Fourth Cup of the Seder. He also prophesied the bread when he said,
16 But you would be fed with the finest of wheat; with honey from the rock I would satisfy you.
14 He grants peace to your borders, and satisfies you with the finest of wheat.
So I'm not sure what you mean by center Communion based on his sermon. Communion is union with God; it's unclear to me how that's accomplished by a sermon (actually we call it a homily, which is a sermon based on the Scripture readings proclaimed). The Word of God is Jesus, not the Bible (John 1:1). Our faith is centered on a Person, not a book (Jesus never wrote a word of Scripture). The Eucharist is the Word of God. While the homily has an important role of explaining the Scripture readings, the real center of worship is not a teaching moment, as important as it may be, but an encounter with the Flesh and Blood of the Living God (see Hebrews 12:18-29, in particular note verse 24).
A note about definitions: A Mass by definition involves the Eucharistic sacrifice (what you term perform[ing] transubstantiation). Without the Eucharist, it is not a Mass.
Asking whether a priest is obliged to perform transubstantiation during a Mass is a bit like asking whether baseball players are obliged to play at a baseball game.
Now it is possible to have a communion service where pre-consecrated Eucharist is distributed to the faithful without a new consecration (this is done, for example, on Good Friday, or if a priest is not available), but this is not a Mass, but a communion service or pre-sanctified liturgy.