It’s hard to say who this man was, as there are not a lot of details in the text or original Greek. The Greek δεῖνα (deina) is as you describe, but does not concretely give clear insights on who he was. In a parallel story recorded in Mark 14:13-15, we are given more details about this man:
Mark 14:13-15 NASB
 And He sent two of His disciples and said to them, “Go into the city, and a man will meet you carrying a pitcher of water; follow him;  and wherever he enters, say to the owner of the house, ‘The Teacher says, “Where is My guest room in which I may eat the Passover with My disciples?”’  And he himself will show you a large upper room furnished and ready; prepare for us there.”
Unfortunately there is nothing further here where we can determine concretely who the man was. The key detail, “a man […] carrying a pitcher of water,” may be tied to some cultural meaning. Jewish men in those days normally did not carry jugs of water. We cannot be sure if this unique characteristic of this man was just in Jesus’ supernatural knowing (confer with the calling of Nathanael in John 1:48), or an arranged signal He had organized earlier. Jesus Himself was a wanted man, and He was looking for a private place to share in His last Passover meal with His disciples. However, I believe we can only suppose further who this man was and how he came to be carrying a jug of water as Jesus foreknew.
This was an interesting detail that I had not thought too much about till now. I’m curious if anyone else has further insights.
 Craig L. Blomberg, Matthew in The New International Commentary, vol. 22, eds. David S. Dockery, et al. (Nashville: Broadman, 1992), 387-388.
 Craig A. Evans, Mark 8:27-16:20 in Word Biblical Commentary, vol. 34B, eds. Bruce. M. Metzger, et al. (Grand Rapids: Zondervan, 1988), 15545-15569, Kindle.