Right-hander Jose Dominguez, who turns 26 next week, had his contract selected from Triple-A El Paso on Friday, and he'll take Johnson's roster spot. There is no timetable for when Johnson could return, but the Padres hope to have a better idea this weekend.
"Frankly, at this point in time, we don't know," said Padres manager Andy Green. "... Hopefully it's nothing severe, but until we get a chance to look at the MRI -- maybe we know by tomorrow what the results are."
Johnson has endured a shaky start to his Padres tenure, as he's posted a 9.15 ERA in four starts while allowing nine home runs. The Padres have lost all four of his outings.
As for Dominguez, it's his first chance with the Padres after he spent parts of three seasons in the big leagues with the Dodgers and Rays. He owns a 3.79 ERA in 27 outings, but has struggled with his control, walking 25 in 35 2/3 frames.
"If he attacks the strike zone and pitches ahead, he's going to be very, very effective," Green said. "He was a guy for us who we needed to take a look at before we bypass and start looking at a crop of younger guys. I think he warrants an opportunity."
The Padres will need to make another roster move before Sunday's game, when Andrew Cashner is slated to return from the disabled list. Green hinted that Dominguez's spot could be safe, and that the club might opt to send down a position player, giving some help to a taxed bullpen.
Clemens arrives in San Diego
Right-hander Paul Clemens arrived in San Diego Friday -- and was given the locker vacated by Fernando Rodney, who was dealt to the Marlins Thursday afternoon.
Clemens will begin his tenure with the Padres as a reliever, but Green said he could develop into a starter. He posted a 4.30 ERA and a 1.21 WHIP in 14 appearances -- all starts -- for Triple-A New Orleans this season.
"I've always preferred starting, but I've done a multitude of roles," Clemens said. "Whatever the Padres feel is the best chance to help them win, I'll go out there and do that job."
On multiple occasions, Green has compared Clemens' curveball to that of Drew Pomeranz, who has been wildly successful with San Diego -- in large part because the club encouraged him to throw the pitch more frequently.
That's the plan with Clemens, who has thrown his hook about 17 percent of the time during parts of three seasons in the big leagues. The Padres want that number to go up -- and don't be surprised if it nearly doubles.
Clemens said he's well aware of the Padres' reputation for allowing struggling pitchers to prosper. Most notably, pitching coach Darren Balsley has helped revive the careers of current Padres starters Tyson Ross, Christian Friedrich and Pomeranz.
"I definitely think this is going to be a very good spot for me," Clemens said. "I've always known I have the stuff to pitch in the big leagues. As far as putting it all together, using all my stuff, I still think I have a little ways to go."
Spangenberg headed to Arizona
Injured second baseman Cory Spangenberg will travel with the club to Arizona next week, where he'll continue rehabbing his strained left quad at the Padres' complex in Peoria.
"It feels good right now," said Spangenberg. "There's no pain, there's no tightness right now. It's looking hopeful."
The 25-year-old second baseman took another step in his recovery Friday when he took ground balls for the first time. He also ran 30-yard dashes at what he equated to 50-60 percent.
"I'm hoping he's off for a rehab assignment before the All-Star break," Green said. "That's what I'm hoping for. He's got to continue to build up."
• According to MLB.com's Jim Callis, the Padres have signed University of Florida center fielder Buddy Reed, their second round pick in last month's Draft. Terms of Reed's signing bonus were not disclosed. With Reed's deal, the Padres have now inked all 13 of their selections from the first 10 rounds of the Draft.
• First-round pick Cal Quantrill made his professional debut Thursday night, tossing two scoreless innings in the Arizona Rookie League. Quantrill, who was appearing in his first game action since Tommy John surgery last March, allowed two hits and struck out two.
• The Padres activated Buddy Baumann from the disabled list Friday and optioned the lefty reliever to Triple-A El Paso -- where he's been pitching for the past month in a rehab stint. Baumann, who was recovering from a lower back strain, has allowed two earned runs on six hits in 12 1/3 innings for the Chihuahuas.
• To make room for Baumann on the 40-man roster, the Padres transferred right-hander Cesar Vargas to the 60-day disabled list. Vargas suffered a flexor strain in his right elbow in May and wasn't projected to return within the next month anyway, making the transaction a "no-risk" move, according to Green.