How does one explain transubstantiation at the Last Supper? If Jesus had not yet sacrificed His human nature, how could he offer his body and blood to the Apostles in the form of bread and wine?
The basic answer is that it is not necessary for Jesus to have sacrificed himself on the Cross in order for transubstantiation to occur.
In transubstantiation, two things happen: (1) The substances (i.e., the ultimate, underlying realities) of bread and wine cease to exist, leaving only the properties detectable by our senses and (2) the substance of Christ's body, blood, soul, and divinity become present.
For neither of these things to happen does Christ have to have offered himself on the Cross. God created all matter out of nothing (Latin, ex nihilo), and he can similarly cause it to return to nothing (ad nihilo = where we get "annihiliate").
Similarly, God can make any object he wants present at any location he wants, including multiple locations simultaneously. This phenomenon, known as multilocation, is possible for Christ and for anything else God chooses.
The Last Supper, Good Friday, And Transubstantiation – Jimmy Akin