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Keith Urban just played his first show since news broke of his split with Nicole Kidman amid a possible new romance ... and the guitarist at the center of the rumors was notably absent -- but not for the reason many assume.

A source with direct knowledge tells TMZ ... Maggie Baugh , who played with Keith's tour in Chicago, sparking romance rumors due to some apparent lyric rewrites ... is not actually a member of his tour band or the opening acts. Getty

Turns out, Maggie was filling in for Natalie Stovall when she was unable to perform in Chicago. Ergo ... she wasn't at last night's show at Giant Center in Hershey, Pennsylvania ... 'cause she wasn't needed -- not due to her possibly bailing over the headlines.

Now, Maggie was also filmed playing with Keith in Vegas

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