"There's many ways to make a roster better, but we decided that if we could find a starting pitcher who could be difference-maker, we were going to go after it," Byrnes said Wednesday.
"[Johnson] has led the league in ERA and has been a dominant pitcher, and we had the opportunity to sign him. We know there's risk, but we're very excited about the upside about what he can bring."
On Wednesday, the Padres and the 29-year-old right-hander agreed to terms on a one-year deal worth $8 million, though Johnson can earn an additional $1.25 million if he makes at least 26 starts.
The deal with Johnson is the largest contract the organization has given a free-agent pitcher since Greg Maddux agreed to a one-year deal worth $10 million in November 2007.
The team also holds a club option of $4 million for 2015 if Johnson is unable to make seven starts, which is essentially risk protection given that he is coming off a rough 2013 season physically that saw him undergo surgery on Oct. 1 to remove bone spurs as well as some loose cartilage in his right elbow.
"He is coming off surgery, so this gives us some protection … in that we at least get another bite of the apple if for whatever reason that this season doesn't go as we hope," Byrnes said.
But Byrnes and Johnson -- who turns 30 on Jan. 31 -- don't anticipate any problems getting ready for the 2014 season. Johnson has already discussed his throwing program, which will begin on Dec. 1, with his new manager, Bud Black.
"We felt as though when we looked at the players we might be able to acquire, Josh's name came to the top of the list. I've seen Josh from the other side. This guy potentially for us brings a lot to the table," Black said.
The Padres believe they are getting a top-of-the-rotation arm in Johnson, who led the National League in ERA (2.30) in 2010 and posted a 3.15 ERA in parts of eight seasons with the Marlins, which a combined WAR of 11 from 2009-10 before a disastrous run with the Blue Jays last season, when he wasn't healthy, had a 6.20 ERA in 16 starts and didn't pitch after Aug. 6.
Johnson did, however, have a career-best strikeout-to-inning ratio (9.2) and maybe a little unlucky (.356 BABIP, batting average on balls in play) during his time in Toronto.
"Every time that you get hurt, you start thinking the worst … or think things you shouldn't think," said Johnson, who was sidelined by inflammation in his triceps, a strained forearm and a knee injury.
"It was frustrating not being able to go out there and not knowing what's causing it. At the end of the year, we figured it out and hopefully we'll keep moving forward."
Johnson, as many pitchers before him, figures to benefit from spacious Petco Park, which, despite some fence modifications before last season, still plays big. There were 21 "new fence" home runs last season at Petco Park, though run scoring was actually down (7.4 runs per game) from 7.7 in 2012. The league average this past season was 7.9 runs per game.
"Every time that you come here [San Diego], you want to stay here," said Johnson, who makes his home in Las Vegas. "The park is great, the city is great … the fans are great. That's pretty much what you want. The staff, the team … it's a perfect fit."
And, as Byrnes said, Johnson is a perfect fit for the starting rotation, which includes Andrew Cashner, Tyson Ross, Ian Kennedy, Eric Stults, with Joe Wieland and Cory Luebke-- who are coming back from Tommy John surgery in 2012 and expected to be ready for Spring Training -- and several other young pitchers who are considered very close.
San Diego pitching coach Darren Balsley, reached on Wednesday at his home in Tennessee, was already excited about getting the chance to work with Johnson.
"If he's healthy and close to what he was a few years ago … when he's on, he's very good. I even thought he was a little underrated at the time," Balsley said.
"There are a lot of things to like. The power, the downhill trajectory of his pitches [Johnson is 6-foot-5], I know our guys had trouble picking up his pitches. That's big, as big as having good overall stuff. And he has deception, too. That's tough for a hitter to figure out."