Van Nistelrooy will be by far the biggest attraction, even just sitting in the stands, when fifth-placed Hamburg, seven points off the pace after 19 matches, host troubled champions VfL Wolfsburg on Friday.
"Hamburg are one of the favourites now," Bayern Munich honorary president Franz Beckenbauer said on Thursday.
"They have acquired a world-class striker in Ruud van Nistelrooy who can singlehandedly decide matches when fit."
Bayern's Dutch winger Arjen Robben agreed that Hamburg, with the addition of van Nistelrooy, could mount a challenge for the Bundesliga title.
"I consider him as one of the best strikers in the world," Robben told reporters.
"Hamburg are a very good team with lots of quality and they are now only going to get better with him."
The 33-year-old Dutchman is trying to get fit after a string of injuries and only a handful of matches in Spain this season. He is expected to start training with the team next week.
Hamburg, who were top of the table until late October, have had to deal with a string of injuries to several key players, including strikers Mladen Petric and Peruvian Paolo Guerrero, who is recuperating from a cruciate ligaments injury.
Van Nistelrooy could not have arrived at a better time, with Guerrero grounded in Peru, his fear of flying having forced him off the plane on several occasions.
Second-placed Bayern, on an eight-match winning streak in all competitions, will attempt to make it seven league wins in a row as they chase undefeated leaders Bayern Leverkusen, two points ahead on 41.
Bayern could provisionally go top for the third straight week with a win against Mainz on Saturday, with French playmaker Franck Ribery expected to play. Leverkusen are in action on Sunday.
Ribery has recovered from a knee injury but has yet to last a full match as he struggles with match fitness.
Leverkusen, who will be without captain Simon Rolfes following knee surgery, entertain lowly Freiburg.
