Professor Stephen Holgate, a respiratory specialist at University Hospital Southampton NHS Foundation Trust, told Sky News the decision to allow the US president to leave the Walter Reed military hospital was "not textbook".
"The disease can get worse very quickly," he said.
"He is still in the first wave. The second is yet to come, when the immune system goes into overdrive. It will probably hit him in two to three days' time."
Some patients with the disease can appear to improve, but then relapse as the immune system swamps the lungs with signalling chemicals - resulting in inflammation.
That can lead to blood clotting and damage to the air sacks that transfer oxygen into the blood.
Mr Trump - who is still infectious - appeared to be breathless upon his return to the White House after a three-day hospital stay.
His doctors had earlier refused to disclose details of CT scans carried out on his lungs.
Prof Holgate told Sky News breathlessness was "a cardinal symptom of lung involvement", adding: "Even people with mild disease have small lesions in their lungs."
Mr Trump is still taking the anti-viral drug remdesivir and the steroid dexamethasone.
But Prof Holgate said it was unusual to prescribe the steroid to a patient who did not need a ventilator, and the value of remdesivir was uncertain.
"The clinical trials are not persuasive," he said. "I'm not at all confident in the treatment."
The White House has a medical suite staffed by a team of doctors and nurses - but Prof Holgate said that it looked "more like a hotel room".
"He does have a place there, but if you need a ventilator you need it quickly. It does not creep up on you," he said.
"The key thing is does he have access to acute medical team and equipment."
Professor Peter Openshaw - a lung specialist at Imperial College London - said Mr Trump's age, gender and obesity would put him at higher risk of a poor outcome.
"The whole thing is unconventional," he said. "It would be unusual for someone to be allowed to go home in those circumstances, but he does have a strong medical team.
"The statistics show even with his risk factors he is likely to survive, but we do not know what the long term consequences will be."