Looking at the flight history above from both flightaware and flightradar24 I have pieced together the following movements. It seems that the plane was already stationed in Kenya but planes aren't easy to track while they move around in that part of Africa.
There are reports that Nebraska will monitor the patient for "up to 2 weeks" while there is a 21 day incubation period for ebola, indicating that the exposure could have occurred a few days ago.
It is curious, given the incubation period and quarantine period, that the plane has been in the region for 10 days and that the flight was diverted. Might be nothing, or it might signal that there were difficulties clearing this flight for entry.
Also worth noting that Goma is a 10 hour trek from the centre of the outbreak in Beni, which is a logistical nightmare with an ebola patient. There are thankfully no symptoms reported in this case, but it is unlikely that anyone showing symptoms would be taken on that 10 hour journey if it is going to take 10 days to clear entry into the US. This is a very different situation than the West Africa outbreak when several foreign HCWs were repatriated.
2018-Dec-29 Dulles to Nebraska
2018-Dec-29 Portugal Diverted to Washington Dulles
2018-Dec-29 Senegal to Azores (Portugal)
28 Dec 2018 Congo (Goma) to Senegal
28 Dec 2018 Kenya to Congo (Goma)
20-27 Dec - unknown
2018-Dec-19 Kenya to Somalia