Rory McIlroy is doubtful for the BMW PGA Championship after suffering from ongoing back problems in the Players Championship at TPC Sawgrass in Florida.
McIlroy revealed he would have an MRI scan on Monday to determine the extent of his back problem, with the world number two admitting the "most important thing" is to be fit for next month's US Open.
The BMW PGA Championship takes place in a fortnight's time and has a prize fund of £5.4million as the first event of the European Tour's Rolex Series.
McIlroy's injury is a combination of the back and rib problem which kept him out for seven weeks at the start of the season, although he was able to add a 71 to his opening 73 to reach halfway at level par, nine behind clubhouse leaders Louis Oosthuizen and Kyle Stanley, both on nine under par.
"I hit a couple of tee shots practising on Monday and just didn't feel right and I've been getting treatment on it," McIlroy said.
"We just hope it's not the same thing, the joint and the rib.
"It feels more like a muscle strain so I'm going to get an MRI scan on Monday after the tournament and I might have to rest for a week or two.
"As long as I get myself right for the US Open, that's the most important thing."
Speaking about a round containing two birdies and one bogey, the newly-married four-time major winner said: "Today it seemed like the golf course should be gettable but no-one really posted any low numbers.
"It was tough to get close to a lot of the pins and I only birdied one of the par fives. Overall it was okay; I can go out tomorrow morning, hopefully post a good score and get myself right back into the tournament."
Masters champion Sergio Garcia and world number one Dustin Johnson were alongside McIlroy on level par after rounds of 71 and 73 respectively.
Ulsterman Graeme McDowell was having more luck, sitting on two under through 15 holes of his second round.
Offaly's Shane Lowry missed the cut after his second round 76 left him on six over par.
Last year's Masters champion Danny Willett is also struggling with his back and could be ruled out of the BMW PGA Championship.
"Hopefully, touch wood," Willett said when asked about his prospects of competing at Wentworth.
"We'll see how things go. I've already limited the schedule down a lot, obviously all of the travelling we've been doing, and everything that's been going on.
"Hopefully, all going well, we get back on. We have a week to do some work and see how it feels."
Willett, who has been hampered by back problems throughout his career, struggled to an opening 79 on Thursday and had covered the back nine at Sawgrass in 40 before calling it a day.
The 29-year-old from Sheffield wrote on Twitter: "Not good to have to withdraw but swinging very poorly is putting a lot of unwanted strain on the back.. body and mind need a rest!!"
Willett added: "Woke up and it was tight. I don't really need to see anyone else to tell me, I've had it many a time before."
Willett has slipped from a career-high of ninth in the world to his current position of 21st after a string of poor results, including becoming the first defending champion since Mike Weir in 2004 to miss the cut at Augusta National.
He also made an early exit from his next event in the RBC Heritage, where he split with long-time caddie Jonathan Smart after the first round.
Meanwhile, England's Matt Wallace extended his lead to five shots in the weather-affected Open de Portugal at Morgado Golf Resort.
Wallace, who carded 10 birdies in a flawless opening 63, added five more in the first 12 holes of his second round yesterday before play was suspended due to darkness, a knock-on effect of the time lost due to Thursday's thunderstorms.
The 27-year-old Londoner, who won five tournaments in succession on the Alps Tour last season and six events in total, started on the back nine and picked up shots on the 10th, 12th, 13th and 16th to reach the turn in 33.
Further birdies on the first and third took Wallace to 16 under par and five ahead of Germany's Sebastian Heisele, who followed his opening 64 by carding three birdies and a bogey in 12 holes on day two.
England's Ashley Chesters and France's Matthieu Pavon had set the clubhouse target on nine under after rounds of 71 and 69 respectively.
Pavon was one under par after nine holes when the first round was suspended and when play resumed at 8.30am yesterday, he made four birdies on the back nine to post a bogey-free 68.