It appears likely Marquis will depart the Keepmoat this summer after being the subject of bids a year ago and also during the January transfer window.
But McCann – who has been given assurances he can reinvest some of the money received in a potential transfer – says Marquis will not be sold until an interested party matches a strict valuation.
"There's been quite a bit of interest in John,” he told the Free Press.
"The club don't need to sell him and John and his representatives are well aware of that.
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"But on the other hand, everyone has a value and Gavin [Baldwin, chief executive] and David [Blunt, chairman] have said if a valuation is met then that helps me in terms of another couple of positions that we're looking at.
"Obviously one of those will be in that area if John does go.
"That money will help us if he does go.
"But at the minute the valuation is not where we think it should be.”
McCann met with Baldwin and Blunt last week and one of the items on the agenda was a strict valuation of Marquis, who netted 26 goals in Rovers’ run to the League One play-offs.
"We have a set valuation and I think it's important.
"He scored 26 goals last year and those type of strikers don't come cheap.
"The valuation will be there and again, the board and David have given me a real sense of belief that we can go and replace John with the targets we have in mind and maybe one or two other areas.
"At the minute it's not been met, we don't have to sell and we'd love to keep John.
"But that's just where we are at the minute."
McCann revealed Marquis at one stage had his heart set on joining League One rivals Sunderland during the January window but changed his mind on several occasions.
The Black Cats reportedly lodged a deadline day bid of £2m for the striker, but Rovers were steadfast in their determination not to sell and leave themselves with no time to find an adequate replacement.
“He's contracted for another year so a conversation I had with John in January was stay and help us get promoted,” McCann said.
“He was upset at the time.
“He didn't want to go, then he did want to go, then he didn't, then he wanted to go to Sunderland but in the end decided he wanted to stay which we were delighted with.
“It was a long day that last day of the transfer window. We had no time to replace him.
“One thing I'm about is if people want to move on and progress their careers then fine.
“Sunderland are a huge club but at the time I thought John could stay and help us get promoted, which is what I spoke to him about.
“For him to finish on so many goals, there's going to be all sorts of clubs chasing him.
“We've got to play it by ear.
”We've already got replacements in line to come in if he does go.”