City make their debut tonight against Napoli.
Giggs admits United struggled with the workload in the early days of the Champions League and feels the fact that City are away from home after every group fixture (against Fulham, Blackburn, United, QPR, Liverpool and Chelsea) could place them under strain.
"City will be a threat because of the quality they've got and their money means they've been able to go out and buy so many top players," Giggs told MUTV. "But I think the test is really the first season in the Champions League. Tottenham found that out to a degree, I think, they started off last season so well but it is tough. We found it tough early on.
"Playing in a Champions League game, which takes so much out of you physically and emotionally, on a Wednesday, and then a tough league game on a Saturday, maybe away - that is the test. It is tough and you've got to have the squad to handle it. We'll just see if City can do what Arsenal, Chelsea and ourselves have done over the years."