The German, who lost his Renault seat at the end of last year, got the unexpected call-up to replace Perez after the Mexican tested positive for coronavirus on Thursday.
"I was actually on my way to the Nurburgring for another racing project when I got the call from Otmar (Szafnauer)," Hulkenberg said. "That was not even 24 hours ago. It all feels a bit surreal."
"I can feel it physically," said Hulkenberg, who has not driven an F1 car since the 2019 season finale.
"And I'll certainly feel it tomorrow and the day after tomorrow as well," he added.
The entire paddock is happy to see Hulkenberg again, but questions are being asked about how Perez caught the virus amid F1's ultra-strict anti-corona measures.
Perez admits he flew to his native Mexico after Hungary to see his injured mother after an accident, but his wife Carola also published photos of a holiday to Porto Cervo, an Italian seaside resort, on Instagram last week.
Those photos have now been deleted.
"Hindsight's a wonderful thing, but there are hotspots all around the world," said Szafnauer when asked about Perez's travels.
But it also emerges that Perez, who is asymptomatic, did not even ask his team for permission to travel abroad between Hungary and Silverstone.
"We've got no clauses in the contract where he's got to ask permission to go back to his family," Szafnauer insists. "There's no issue with that."
But with measures as strict as they are at races, he admitted that Formula 1 might need to think about tweaking the guidelines to prevent further infections.
"In hindsight, which is a wonderful thing, perhaps we should look at that and say that throughout the season, you stay within your bubble," said Szafnauer.
"But I don't think Checo did anything wrong going back to his family. I think it's no different to Ferrari going back to Italy, for example."
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