Aegean cannot do anything for you as this is a United ticket. They have rebooked you so they are your point of contact regarding the rebooking proces (including selecting other flights).

The 45 minute connection is within MCT so you should be able to make it. However it would be tight, and everything needs to be running smoothly. On the other hand, they would not be selling this connection if a large portion of pax are misconnecting, as in every such case they would need to compensate the pax.

Moving this to the United subforum for rebooking options with United.