And a gentle word in the striker's ear may have just done the trick.
Marquis scored either side of the break to put Rovers in command against the battling side from West Yorkshire.
He rifled one into the top cornerÂ with his right foot from the edge of the area before a drilled left footed effort capped a sweeping team move to double their advantage.
"John was excellent," McCann said. "He's been excellent this season. He's a goalscorer.
"I had a feeling he was going to score with his left foot.
'I said to him on Thursday and Friday to get some left footers down the side of people.
'Especially with Bradford playing a three at the back, there was going to be opportunities to get down the sides of them.
'And his finish was excellent.'
Rovers secured a third consecutive win with their triumph over Bradford, but were made to work for it, particularly after substitute George Miller pulled a goal back for the Bantams nine minutes from time.
Though he was reluctant to dwell on the the manner of the performance from his side, McCann declared them '˜iffy' afterwards but praised their show of resolve.
"We won the game so it's irrelevant how we performed really because it's all about the result," he said.
"In terms of our performance, I thought we were iffy.
"We had some spells in the game where we dominated. We had some spells where we had to dig in.
"We had spells where we could have scored again but didn't. Mallik Wilks could have put us 3-0 up.
"Then we end up making it a bit tough for ourselves and end up having to defend by putting bodies on the line in the last ten or 15 minutes.
"We've shown that resolve this season. We showed it against Luton.
"I'm delighted with the result. It's a first class performance against a team that I don't see being down where they are for too long, especially with the front three they've got.
"They can cause anyone problems in this league and they did for us.
"We're delighted that we defended it to a man."