"It was tennis. I would rather say it was anything apart from tennis because it sounds kind of dumb but that's the way it went," said the Colombian at a Spanish Grand Prix news conference.
There had been considerable speculation in Formula One circles that Montoya, a keen motocross fan and not a noted tennis player, had suffered his hairline fracture of the shoulder by falling off a bike.
Montoya will make his return in Sunday's race, subject to final medical approval on Friday, after missing the Bahrain and San Marino Grands Prix.
"I'm feeling great to be honest," he said. "You know, it's been kind of weird to sit down at home and watch the last two races on TV.
"In a way, I'm very pleased. Both of the third drivers did a fantastic job for the team so it is nice to see the team still scoring points and everything. But it's great to be back."
Austrian Alexander Wurz took third place at Imola as a stand-in, following the disqualification of both BAR drivers, while Spaniard Pedro de la Rosa was fifth in Bahrain.
McLaren are third in the championship after four races.
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