Bruce, speaking to The Telegraph, has revealed there is "widespread opposition" to Project Restart getting off on June 12.
He believes players need longer to get back up to fitness after eight weeks off due to the coronavirus pandemic.
Speaking to the newspaper, the 59-year-old said: “We've listened to what has been said and, with the precautions taken, we will get back to work and start phase one.
“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 time to get these players into shape or they are just going to fall down like a pack of cards.
"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."