Johnson, whose eight-year unbeaten run over one lap came to an end in Wednesday's Paris Grand Prix, has pulled out of the Sheffield meeting and Wednesday's Lausanne Grand Prix in order to fly home to the United States and regain full fitness.
"The next time he puts his foot to the track he will be right and I'll be amazed if he does not run 43 seconds again this year," Black said yesterday.
Johnson recorded his slowest time in a decade - 45.76sec - as he trailed in fifth in a race won by his American colleague, Antonio Pettigrew.
Black, second to Johnson in last year's Olympic 400m final, added: "There must be something wrong. For an athlete of Michael's stature to run as badly as that, something is seriously wrong. It's like me running 47 seconds. It just shouldn't happen."
The double Atlanta gold medallist missed two weeks' training after injuring a quadricep muscle in his 150 metres head-to-head challenge with Canada's 100 metres world record holder, Donovan Bailey, in Toronto three weeks ago.
"I'd like to think I could miss two months training and still run better than that," Black said. "Michael is certainly doing the right thing by going home to sort things out. It is sad for Sheffield, because we went to a lot of trouble to persuade Michael to run in Britain again.
"But an athlete as great as he is should not subject himself to the possibility of another poor race." Black, whose assumption of the British captaincy now looks certain following the assurances of Linford Christie's management group, Nuff Respect, that their man would not be running in Britain's World Championship trials next month, now faces a domestic challenge in Sheffield.
He will line up against his three team-mates in the successful 4x400m relay squad from the European Cup triumph in Munich: Jamie Baulch, Mark Richardson and Iwan Thomas.
"It's great for British 400 metre running to have those three youngsters yapping at my heels," Black added. "It means I'm not even certain of making it into the World Championship team. I am British captain and Olympic silver medallist and no athlete of that stature has that sort of pressure within his own country."
Johnson will run in Britain before the end of the season, his manager, Brad Hunt, promised yesterday.
"Michael now knows he came back earlier than he should have. He needed another couple of weeks' training," Hunt said.
"I think the people in Sheffield will understand that he does not want to compete if he knows he is only going to perform to 85 per cent of his ability."
"He will be back in Britain, probably for the meetings at Gateshead and Crystal Palace before the end of the season."Reuse content