Former South Africa captain Graeme Smith has questioned the fitness levels of Vernon Philander after the seamer was ruled out of the fourth and final Test between the Proteas and England.
Philander battled a viral infection during the third Test at The Oval and will play no part in the series finale - which begun at Old Trafford on Friday - after being hampered by lower back spasms.
Smith told ESPNCricinfo: "This whole series, it's been a struggle to keep Vernon on the field. It gets frustrating when you've got a senior player and an outstanding performer like him and fitness is becoming an issue.
"It raises questions about whether he is fit enough to be picked in the first place.
"He needs to look at that, he needs to do a lot more work in his time away from the team to make sure this doesn't keep happening."
Smith, the skipper of South Africa when Philander first broke into the national side, continued: "Sometimes you need to be a little bit harder on Vern. His skill levels are there but you need to get him into the contest sometimes.
"One of the big issues for him is fitness and he does need to take a look at it. He is at that phase of his career where if he doesn't, he could fade away pretty quickly.
"It makes it very difficult for Faf [du Plessis] to build a team. You can't afford Vernon to not be finishing a series."