The Football Association of Wales says in a statement the injury was suffered before Bale joined the Wales squad.
The 21-year-old Tottenham Hotspur winger did not train on Wednesday.
"He did the warm up on Tuesday, we took him off and he went for a scan on Wednesday and he had an injury there," said manager Gary Speed.
"It was an injury from last week and we do our utmost here to make sure we look after the players. Gareth is the most important person in this scenario."
The FAW released a statement on Thursday explained the problem with Bale's injury.
It said: "After playing 90 minutes on Saturday he felt some muscle tightness. Initially, this was expected to be muscle soreness after playing his first game.
"On joining up with the Welsh squad he was kept off his feet in training for two days to recover.
"As he still felt tight in the warm-up on Tuesday, Gareth was pulled out of the session.
"He did not train at all on Wednesday and was sent for a scan.
"The scan revealed that Gareth Bale picked up his injury last week."
Despite Bale being highlighted as one of the key players in Wales bid to beat England in Cardiff, Speed stressed that he is more interested in the long term rather than the short term benefits of risking him on Saturday.
"The next three and a half years are more important than the next three and a half days," he added.
"We want to be successful and the best way we can be successful is to get our players out on the pitch in the best shape they can be."
Bale has only recently returned to action for Spurs after a six-week lay-off with a back injury.
His absence is a major blow to boss Gary Speed's preparations but a boost to England, who see Bale as one of their main threats.
"Gareth is top class and he can really hurt you - it's a shame at times he is not English," said England striker and Tottenham team-mate Peter Crouch.
Speed had hoped he could unleash "match winners" Bale and Arsenal midfielder Aaron Ramsey on Fabio Capello's side.
Thu Jan 21 2021, 20:01 by BazSpur
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