'We've tested him multiple times, including the pre-event test for this race.
'He hasn't been feeling great since Russia, ' Szafnauer said.
"I think we messed up a bit with our strategy, we thought we had a bit more pace and we tried the medium in Q1 which meant we lost basically a set for Q3", he said.
"And he doesn't have the classic COVID-19 symptoms".
Super-sub Hulkenberg, who replaced Stroll without any practice this weekend, was voted as F1's Driver of the Day on Sunday after rising from 12 spots from 20th on the grid, while Vettel was only 11th of 15 finishers for Ferrari.
"We're just thankful he got within the 107 per cent rule, that's what we wanted to do so he could race", said Szafnauer. He just doesn't feel well in himself and last night he had an upset stomach. "I didn't know what was happening, so I just hope health-wise Lance is okay and we can have him back soon".
Team principal Otmar Szafnauer said Stroll had been unwell since he left the last race in Russian Federation two weeks ago with "flu-like symptoms", but had returned "three or four" negative COVID-19 tests since then. "After the safety auto I was able to pressure Daniel and I nearly got him into turn 4 but I wasn't able to make it stick". "Obviously there's still a long way to go in the season but I wish the team all the best in the coming races".
Perez conceded that the minimal running conducted will make Sunday's grand prix "an unknown race", but said he wasn't overly optimistic. He says "Hulkenberg hurry, we need you here".
It marks Hulkenberg's third Formula One appearance this season.
As a result, the German qualified last and two tenths of a second down on the next slowest driver in Kimi Raikkonen, but Szafneuer believes he can improve throughout the race to secure a top 10 finish.
'We made some changes to the vehicle since he drove it last.
"It felt quite different again to Silverstone obviously, completely different circuit", he said.
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