Theophylact of Ohrid

And He took the cup, and gave thanks, and gave it to them, saying, Drink of it, all of you; for this is My Blood of the New Testament, which is shed for many for the remission of sins. Just as the Old Testament had sacrificial slaughter and blood, so too the New Testament has Blood and slaying. …

Commentary on Matthew ↗

Cyprian

The cup of the Lord is not water only, or wine only, but the two are mixed; so the Lord's Body cannot be either flour only, or water only, but the two are combined.l

Catena Aurea by Aquinas ↗

Ambrose of Milan

If Melchisedech offered bread and wine, what means this mixing of water? Hear the reason. Moses struck the rock, and the rock gave forth abundance of water, but that rock was Christ. Also one of the soldiers with his spear pierced Christ's side, and out of His side flowed water and blood, the water to cleanse, the blood to redeem.m

Catena Aurea by Aquinas ↗

Thomas Aquinas

Above, the institution of the new sacrament was treated with regard to the sacrament of the body of the Lord; here the institution of the same is treated with regard to the sacrament of the blood: and concerning this he does two things. First, the actions of Christ are set forth; second, his words, at drink ye all of this. Concerning the first, three actions are set forth. First, that he took the chalice; second, that he gave thanks; third, that he gave it to the disciples. …

Commentary on Matthew ↗

Glossa Ordinaria

As the refreshment of the body is wrought by means of meat and drink, so under the form of meat and drink the Lord has provided for us spiritual refreshment. And it was suitable that for the showing forth the Lord's Passion this Sacrament should be instituted under both kinds. For in His Passion He shed His Blood, and so His Blood was separated from His Body. It behoved therefore, that for representation of His Passion, bread and wine should be separately set forth, which are the Sacrament of the Body and Blood. …

Catena Aurea by Aquinas ↗