Hartley, 26, was found guilty of biting the finger of Ireland flanker Stephen Ferris in England's 30-9 Six Nations victory at Twickenham. The independent disciplinary panel ruled Hartley's reaction to finding Ferris' finger inadvertently in or near his mouth was "not justifiable" and banned him until May 14.
Harlequins flanker Chris Robshaw was made England captain for the Six Nations but Rowntree told the disciplinary panel that Hartley "was very close to having been made captain of the national team and might well soon become captain".
Hartley could have faced a much lengthier suspension given he was banned for six months in 2007 for eye-gouging Wasps players Jonny O'Connor and James Haskell in the same match.
But the panel decided to "ignore" the gouging incident - and they reduced the ban from a potential 12 weeks - following "glowing" character references from Rowntree and Northampton director of rugby Jim Mallinder.
Hartley's eight-week suspension means he will be available for the Aviva Premiership final at the end of May, if Northampton qualify, and for England's summer tour of South Africa.
Rowntree, the England forwards coach, insisted on attending the hearing to speak on Hartley's behalf and detailed the "massive change" he has seen in the player since his "struggle in 2007".
The full written judgement details Rowntree describing Hartley as "a beacon and a leader" during England's recent troubled times and he added "no-one had more credibility with his peers".
Rowntree finished by saying Hartley's reputation is "completely unfounded" and that "if you look at the many videotapes of the matches in which he has played you will see a man of humility and respect".