The UEFA Women's Champions League returns Tuesday, with Barcelona as reigning champions after beating Lyon in May. An astonishing transfer window has shifted the landscape, and here is one X-factor for each team.

Barcelona: Patri Guijarro, CM. With Alexia Putellas and Salma Paralluelo gone, Patri is the glue in midfield, covering ground and recovering possession. Nicknamed "The Octopus," her selfless work allows Barcelona's attack to flood forward.

Real Madrid: Linda Caicedo, FW. The Colombia international can decide big games with her one-vs.-one ability and counter-attacking threat. Caroline Weir's departure increases her importance.

Manchester City: Yui Hasegawa, CM. The Japan international is City's heartbeat, orchestrating from midfield with outstanding positional awareness.

Chelsea: Lauren James, AM/FW. Despite signing Katie Mc Cabe, Manaka Matsukubo and Melvine Malard, getting James back to her best could have the biggest impact. A fit James would be a huge benefit.