RORY McIlroy has revealed he requires an MRI scan to determine the extent of a back injury which could rule him out of Europe's biggest event.
The newlywed world No. 2, who missed almost two months of action at the start of this year after picking up a rib stress fracture, will undergo a scan on Monday after ongoing problems during the Players Championship.
Despite the setback he was still 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 could now be forced to withdraw from the BMW PGA Championship which 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. He won the event in 2014 and while he may be required to rest a 'muscle strain', he admitted the "most important thing" is to be fit for June's US Open.
"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 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.