The 25-year-old started out at TPC Scottsdale on Friday morning with his usual putting warm-up, but started to feel considerable pain after trying to swing in the practice range.
Matsuyama had not withdrawn in the middle of a tournament since The Honda Classic in 2016, when he was pained by a right hip flexor.
"We're in a good spot going into the weekend", he added.
Westwood won the Malaysian Open here at Saujana Golf and Country Club 21 years ago, and was also the club's touring pro.
"I just feel like I'm comfortable on this golf course, I've had quite a few rounds here and there's been a lot of good ones", said Fowler, adding that his plan for Saturday was to "just keep the gas pedal down". He then bogeyed the 16th, but finished his opening nine with birdies on 17 and 18. "Obviously you can't win it the first couple days, but you can obviously take your self out of it". "I was a little more nervous than I was at the Masters when I was an amateur, I'll tell you that", said Bryson DeChambeau.
Despite his status as a fan favorite, and a healthy world ranking of seventh, Fowler knows he has let too many tournaments slip away, with 12 career runner-up finishes on the PGA Tour to go with his four official victories. "I was a little disappointed in some of the stuff that was said and I don't want much negativity - the normal boos for missing a green, that's fine, but leave the heckling to a minimum and make it fun, support the guys out playing".