The move was confirmed by his father, who said in a Tweet that the season-long move would be be great for his son: "He will get time on the pitch!"
The 19-year-old Green will be the fifth American to join HSV, one of Germany's traditional powers but in trouble now of staying afloat.
Americans at Hamburg:
Paul Caligiuri (1987-88)
Peter Woodring (1992-94)
Benny Feilhaber (2006-07)
Julian Green (2014-)
Neither Hamburg nor Bayern, the reigning Bundesliga champion, has confirmed the deal, but it must be finalized before Monday night, the end of the German summer transfer window.
Green watched from the bench in Bayern's first two games, but he is still looking to make his Bundesliga debut. His only competitive appearance was all of two minutes at the end of a UEFA Champions League game in 2013.
The competition for playing time will be much less at Hamburg than it would be at Bayern, whose squad includes six German World Cup champions. Green is one of 13 current Bayern players who were on World Cup rosters. That doesn't include the likes of Robert Lewandowski, Franck Ribery and David Alaba, who did not go to Brazil.
Hamburg has won six German league titles -- three since the formation of the Bundesliga -- and the European Cup in 1983. It has been around so long that it is nicknamed der Dinosaurier (the Dinosaur) -- it is the only club to have played in the Bundesliga in each of its 52 seasons -- but it was almost relegated for the first time at the end of the 2013-14 season. It needed to win a promotion-relegation playoff series against Greuther Fuerth to stay afloat.
The recent problems are nothing new. Hamburg has not played in the Champions League since 2007 and its 16th-place finish in 2014 came only two years after it finished 15th. When Mirko Slomka was hired as coach in February, he became the 12th HSV coach in the last decade.
Earlier in the year, the club's chairman, Carl-Edgar Jarchow, announced Hamburg had debts of around 100 million euros ($135 million) and it was adopting the "HSV Plus" plan to restructure and attract new investors. Its poor financial situation forced HSV to sell its best player, 20-year-old German-born Turkish international Hakan Calhanoglu, to Bayer Leverkusen.
The news of Green's impending move came the same day HSV opened its home season with a 3-0 loss to tiny Paderborn. It had played to a 0-0 tie at FC Cologne a week earlier in the season opener, meaning it has opened the 2014-15 season against the two promoted clubs and has yet to score.
That alone will ensure Green will get a chance at der Dinosaurier.
After discussion & consultation w/Coach Klinsmann, Julian’s loan 2 Hamburg SV is a great move 4 him! He will get time on the pitch! #usmnt— JGreen, M.P.A. (@jgreenSTPA) August 30, 2014