"It would [mess up someone's swing] in a lot of cases, but Bryce is swinging as hard as he can all the time anyway," Johnson said as he cracked a smile.
Harper was selected by the Mets' David Wright to be one of the National League's representative's in the Home Run Derby, which will take place on Monday on ESPN at 8 p.m. ET at Citi Field.
Harper, 20, is making his second All-Star appearance in as many seasons, and the outfielder entered Tuesday with 13 homers in 52 games this season.
"He's as strong as anybody there, so why not?" Johnson said.
Harper, last season's NL Rookie of the Year, joins Wright and the Rockies' Carlos Gonzalez and Michael Cuddyer as the Senior Circuit's participants in the event. Johnson said he did not know if Harper had picked who would throw to him during the competition.
Stephen Pianovich is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.