The Spanish midfielder moved to Stamford Bridge from boyhood club Atletico Madrid on deadline day.
"When I played with Atlético happily, I did it with 17," Saul said of his number with the Blues.
"They gave me the opportunity to play with this number and I hope it brings me a lot of happiness."
The 26-year-old has signed on a season-long loan deal with the option to buy next summer.
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