Newcastle manager Steve Bruce says Premier League players are unlikely to be ready to resume the season until the end of June, adding an earlier start would see them "fall down like a pack of cards".
Provisional plans for Project Restart were for a June 12 resumption, though that looks to have hit a stumbling block with disagreements over playing at neutral venues and concerns over player safety.
A June 19 date has been discussed, but Bruce says that players will not be match fit until the end of the month, having been inactive since the middle of March.
"We've listened to what has been said and, with the precautions taken, we will get back to work and start phase one," Bruce said in the Sunday Telegraph. "Can we get to that stage where we can have all the players on the pitch together? We don't know yet.
"Phase one will be mainly fitness work, four or five players on a pitch. Let's see how we get on.
"You have to remember that they have had eight weeks off and that is probably the longest break some of these players have had in their careers.
"If we were doing pre-season, we would have six weeks and probably have six friendly games to get them ready for the first league match.
"So, we need a good stretch of time here to get them ready. Certainly once we get into stage two, that is vitally important. We need enough preparation to get these players into shape or they are just going to fall down like a pack of cards.
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"Most of the managers have the same concerns. We would need at least six weeks. I don't see how we can play games until the back end of June."
Bruce said that, providing all of his players and staff return negative tests, Newcastle will start group training on Tuesday.
"Certainly, the measures being put in place, you're probably more at risk going to the supermarket or putting petrol in your car," he added. "We are in a fortunate position, we can get tested every three days.
"We will be tested on Sunday, every player and every member of staff. Once we get the results back, if everybody is OK, we are pencilled to start training again at 2pm on Tuesday afternoon."
On-loan defender Danny Rose will be part of the group, even though he expressed his views on returning in no uncertain terms earlier this week.
He said that "football shouldn't even be spoke about coming back" until the numbers of people being infected have "dropped massively".
Bruce added: "Danny Rose expressed his concerns, but I've spoken to him and he is coming back as well. I don't think there is a footballer out there who has done more for the NHS throughout this than Danny.
"If a player refuses to come in or play, I have sympathy for that and would understand it. Most of my players are happy with it.
"There was a captain's meeting as well and, as a squad, they are happy with the measures that are being put in place. You have to respect everybody's personal views on it."
