Justin Rose believes Dustin Johnson's absence from the Ryder Cup will hamper the United States as they look to regain the trophy at Gleneagles.

Johnson announced on Thursday he is to take a break from golf and seek "professional help for personal challenges."

It means he will not contest next week's US PGA Championship at Valhalla, after which the nine automatic qualifiers for Tom Watson's team will be finalised. The 30-year-old was fifth in the standings and looked sure to make his third consecutive appearance in the event at the end of September.

The world number 16 won all three of his matches in a losing cause at Medinah in 2012 and Rose - who partnered Martin Kaymer to a 3 and 2 defeat against Johnson and Matt Kuchar in the Friday fourballs - said: "He's not going to play Ryder Cup. That will be a blow for the American team.

"I just wish him well, whatever he's facing right now. Can't be easy for him. I wish him all the best. He's a friend of mine but I didn't really know that that was on the cards."

Zach Johnson, Johnson's ­Ryder Cup ­team-mate, was shocked to hear the news at the first round of the WGC-Bridgestone Invitational, which his namesake pulled out of earlier this week.

Former Masters champion Johnson said: "I feel for him. I feel for his family. I feel for his team, those that are associated with him, because they've done a lot of good work this year.

"More than anything, I pray that his well-being will be on the mend. If anything, I just hope this is the start of something really, really positive. I don't know what happened. I don't know what the specifics are, nor should I know, that's none of my business.

"I want him on the Ryder Cup team but that's my selfish side coming out.

Tiger Woods said: "You always want guys with that kind of power to play on any team. I've been his partner in the Presidents Cup in Australia, and the fire power that he has, it's pretty cool."

Johnson, joint fourth in the US Open and joint 12th in the Open Championship this season, released a statement on Thursday which read: "I am taking a leave of absence from professional golf, effective immediately. I will use this time to seek professional help for personal challenges I have faced.

"By committing the time and resources necessary to improve my mental health, physical well-being and emotional foundation, I am confident that I will be better equipped to fulfil my potential and become a consistent champion."