The rotation, as it stands, is Andrew Cashner, Ian Kennedy and Tyson Ross. Eric Stults was pegged to be the No. 5 starter, but would likely get pushed to No. 4 now.
The Padres wouldn't need a fifth starter until April 8.
Wisler, a 21-year-old right-hander, got the start for Johnson on Sunday against the Rangers and allowed five runs on five hits in 2 2/3 innings. In his first 7 2/3 innings this spring, Wisler's ERA was at 3.53, but it jumped to 6.97 on Sunday. He said he struggled with his command against the Rangers, especially to left-handed hitters, but felt no additional pressure to win a job.
"I'm not trying to worry about that," Wisler said.
He's rated by MLB.com as the fifth-best prospect in the Padres' organization.
"It's a young pitcher that has a lot of poise in his game," said Padres manager Bud Black. "He's calm. He's got a good head on his shoulders. He thinks the right things, and in-game, there's a focus and a calmness to his personality that lends itself well to the stressors of pitching in a Major League game.
"He does a lot of things well. For a 21-year-old, he's ahead of the game in a lot of areas."
Black said he would like to see Wisler continue his work on his changeup, regardless of where he opens the season.
Erlin, a 23-year-old left-hander, might have a couple of advantages over Wisler. First, Erlin's already on the team's 40-man roster. He also has big league service time, as he tossed 54 2/3 innings a year ago in 11 appearances, including nine starts, with the Padres. He has a 2.45 ERA in 7 1/3 innings this spring.
"He's been around the big leagues, he has been around us and he's been around our players. When he pitches in a Major League game, it's not new to him," Black said. "Robbie has pitched in Fenway Park, against the Washington Nationals and he's pitched at Wrigley Field. He's done all these things. He has checked some boxes off. Now it's a matter of performing.
"There's a trust factor there with Robbie."
It doesn't appear the Padres will seek outside help to fill Johnson's spot in the rotation.
"Talking to [general manager Josh Byrnes], probably not. We have enough quality pitching to fill out our 12-man staff, our Triple-A staff and our Double-A staff," Black said.
The manager also indicated that right-handed pitcher Donn Roach is strictly a candidate for a bullpen job at the big league level at this point. If he has to go back to the Minor Leagues, Roach will be a starter.
Johnson talked to reporters Sunday for the first time and admitted to being equal parts devastated and perplexed about this injury, which was confirmed by an MRI exam.
"It was crazy. I don't know how to explain it," the veteran righty said. "I didn't feel anything on the inside [of the elbow] during the start [on Wednesday], and then I wake up the next day and it's a little sore. The swelling is the thing that really jumped out at me."
"I don't know if it was the volume [of throwing] or what it was. But it was definitely a surprise."
Johnson had two disabled list stints with the Blue Jays last season, one for right triceps inflammation. He then landed on the DL again in August for the rest of the season with a strained right forearm.
"It feels sort of similar [to the 2013 injury], but not as bad. Last year, it felt extremely sore everywhere," Johnson said.
The righty said he will likely not do anything for a period of 10 to 14 days before he's examined again, but if the forearm is feeling better before then, he could visit the team doctor.
Despite his health history, Johnson said he didn't see this coming. He had surgery in October to remove bone spurs and started his throwing program in December on time. He moved through spring without issue, until last week.
"It couldn't have gone any better," Johnson said. "You expect to have some soreness here and there, and I never really had soreness at all. Then, all of a sudden, it pops up like that.
"[Black] asked me how I felt, and I told him I was devastated. It's a huge setback. We were moving on the right path. I heard [Cameron] Maybin talking the other day to one of the guys saying, 'No negativity in here.' I've got to stay positive and not bring anyone else down."