Marler, 21, has caught the eye for Quins in more ways than one, but Rowntree is interested less in his multi-coloured Mohican hairstyle and more in his ability.
The former South Africa prop Cobus Visagie said recently that Marler was talented but lacked the technique to warrant an immediate England call-up. Rowntree disagrees.
"Now is the perfect time to get Joe Marler involved in international rugby," said the former England and Leicester prop.
"It was the same with Alex Corbisiero a year ago. Joe's got great talents but we've got to find consistency with him and that's the same with a lot of young players coming into the Test arena.
"We've got to find the consistency to marry up to Joe's unique talents. I am confident in Joe."
Asked what was unique about him, Rowntree added: "He is an explosive prop forward because of his carrying game and his defensive game.
"Now we've got to develop the consistency at scrum time, which you have to have in the Test arena. That's my job. But having those talents available to us, I'm excited about."
Rowntree dismissed suggestions that Marler had been warned to tone down the hair-style or he would not be picked by England.
"I've never said he won't play for England unless he gets his hair cut. That is irrelevant for me," Rowntree said. "I just look down!"
Marler edged out Gloucester prop Nick Wood from the senior squad and he will compete with Corbisiero to be England's starting loose-head in the Six Nations.
In the back row, Northampton's Tom Wood is awaiting the results of a scan on his foot before England know whether he will be to face Scotland on February 4.
England's loose-forward options are bountiful, but Rowntree admitted that producing a genuine ball-poaching openside flanker is now a priority.
New Zealand's Richie McCaw, Australia's David Pocock and Wales captain Sam Warburton proved the value of having a player of that ilk during the World Cup.
Wood, Chris Robshaw, Tom Croft, Calum Clark and Phil Dowson can play across the back row, but Rowntree acknowledged that England need to develop an openside specialist.
The Saxons squad features Saracens' Andy Saull, Exeter's Tom Johnson and London Irish flanker Jamie Gibson, while Worcester's Matt Kvesic is banging on the door.
"You can't argue with how Tom Wood has played in the number seven shirt for Northampton this season," Rowntree said.
"He had a great Six Nations last year. We all know his frustrations at the World Cup. He's playing well at the moment.
"Chris Robshaw has probably been the form back row in the Premiership over the last few months.
"There's a couple of good sevens there and Dowson has played seven too. I'm sure Tom Croft could play seven too so we have enough good cover there.
"We know what Saully can do, we have been looking at him for a long time. We are looking to develop that type of player to bring into a senior squad.
"We haven't got a natural groundhog or ball-stealing seven so we're looking to develop one in the Saxons. Saully fits the bill, so does Matt."
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