The teenager was last seen alive in the Brandywell area of the city on 11 March.
His family had been searching the river since his disappearance.
Dean's uncle, Paddy McDaid, said the family were "holding up well, given the circumstances".
"There were mixed emotions this morning when we got the word," he told BBC Radio Foyle.
"There was happiness that Dean had been found, but at the same time we're thinking about what's coming ahead now in the next 24 to 48 hours right up to his burial.
"Everyone that has helped us out along the way, we've taken great comfort from them.
"People we didn't even know put their own daily lives on hold to come and help us."
The body of another man, Jack Glenn, was recovered from the River Foyle in March.
A joint search had taken place along the banks of the River Foyle for both men for a number of weeks.
Jack's grandfather said he felt a huge sense of "relief" for the Millar family.
"It's a bittersweet moment, but at least they have his body now and they can lay him to rest," he told the BBC's Evening Extra programme.
"I have been talking to Dean's grandparents - there has been contact between the families and a sharing of grief.
"The mothers met up on the banks of the river and they shared their common goal - to get both their sons back out of the water. Thankfully, now that's been done."
The police have confirmed that the body recovered from the River Foyle in Londonderry on Tuesday is that of 18-year-old Dean Millar.