"Al-Jazeera's allegations are very concerning, particularly as it relates to the NFL's and MLB's testing programs," Howman said in a statement.
While the documentary's makers and its critics have clashed over the validity of the allegations, the report has nevertheless pushed concerns of illegal performance enhancements in America's most popular sports leagues into the spotlight once again.
Athletics, cycling and baseball have all suffered huge embarrassment over doping in the past decade and Al-Jazeera's report, at the very least, has highlighted long-running differences between WADA and NFL in their anti-doping strategy.
"While the NFL and the MLB are not signatories to the World Anti-Doping Code, in recent years WADA has been working with them and other professional leagues in the United States to try to bring them closer to WADA's program," Howman said.
"In particular with the NFL, we have been offering guidance to enhance, and increase the transparency of, their testing program."
NFL spokesman Brian McCarthy told Reuters: "Regarding Mr. Howman's statement, we have valued our long-standing association with WADA and look forward to continuing to work closely with the organisation to improve the effectiveness of all anti-doping programs."
In August 2011, the NFL became the first major US professional sports league to use blood testing for HGH, bringing the league closer to international standards.
Though WADA welcomed the move at the time, it has since repeatedly called for much closer liaising with the league and the NFL Players Association (NFLPA).
According to WADA, the NFLPA has long been concerned that football players could possess different threshold levels of HGH than other athletes while the players' union has not trusted WADA over the validity and fairness of its HGH testing.
"Any investigation by WADA should begin with an independent one into their own practices, including the scientific basis for their tests and governance," George Atallah, the NFLPA's assistant executive director of external affairs, told Reuters.
"We rejected WADA participation into our drug policies precisely because they failed to be transparent with us over these very issues. Our union is working closely with UNI Global Union to develop strong and fair standards for drug testing."
The Al-Jazeera report claimed that Manning took human growth hormone following neck surgery in 2011, and on Sunday the football player acknowledged he visited a clinic that allegedly supplied the banned substance.
However, Manning, who missed the entire 2011 National Football League season with a serious neck injury, strenuously denied the claim made by the news network's report that he had used human growth hormone.
"As it relates to the particular allegations by Al-Jazeera, WADA expects that they will be carefully investigated by the relevant authorities and that, if warranted, necessary and appropriate steps would be taken," said Howman.
The NFL collective bargaining agreement, ratified in 2011, banned human growth hormone but players were not tested for the banned substance until 2014. No NFL player has tested positive for HGH.
