After sitting out loan club Crystal Palace's 3-0 defeat against his parent club on the first day of the season, Gallagher has started each of the following 11 Premier League games, during which Palace have lost just once.
While the midfielder has impressed at Selhurst Park, Sidwell revealed Gallagher had set his sight on forcing his way into Thomas Tuchel's team ahead of next season.
"I remember Rio [Ferdinand] telling me he did an interview with him [Gallagher] not long ago," Sidwell, who played for the Blues in 2007-08, told the Daily Express.
"And he said to Rio: 'When you speak about me, make sure that I state that I want to go back to Chelsea. I'm not here in the shop window to get a move. I'm doing this to show them what I can do to go back to Chelsea'."
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