Rory McIlroy is set to have an MRI scan on rib and back injuries on Monday, with the world number two admitting the "most important thing" is to be fit for June's US Open.
He aggravated a long-standing back problem in practice ahead of the Players Championship this week.
The BMW PGA Championship takes place in a fortnight's time and has a prize fund of €6.4m 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, just five behind clubhouse leaders Rafa Cabrera Bello and David Hearn.
McIlroy said: "I hit a couple of tee shots practising on Monday and just didn't feel right and I've been getting treatment on it. 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't believe I'm only five off the lead and 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.