The 29-year-old forward was replaced in the 80th minute of Real Madrid’s 1-0 La Liga loss at Alaves on Saturday.
“Gareth has got a little bit of muscle fatigue and he won’t be involved against Spain,” Wales manager Ryan Giggs confirmed.
“We don’t want to take any risks and we are monitoring it every day.”
Bale sat out training with the Wales squad in Cardiff on Monday and Tuesday.
Although he will miss the glamour friendly with Spain at Cardiff’s Principality Stadium on Thursday, Giggs remains hopeful Bale could play in the competitive game with the Republic in Dublin five days later.
“I would say he is 50-50 at the minute for the Ireland game,” Giggs said.
“He had a scan last week, a couple of weeks ago with Real Madrid. We rescanned it and have done everything we can to get as much information. He feels good but we have to weigh up the risk whether he can be part of the game on Tuesday.
“There was a little bit on both scans – but there’s always something. We’re treating the symptoms but it’s nothing to worry about. Either way, I expect him to be fit for Madrid’s next game.
“I don’t think I was ever 100% playing. When you have niggly injuries it’s maybe at the back of your mind until you’re flying again, but I’m not worried and Gareth isn’t worried.
“It’s something of nothing but it’s a short turnaround and we’ll take no chances. But if he’s available he’s available. The medical team are constantly in dialogue with Madrid like they are with every team.
“Playing in Spain he wants to be a part of it [the friendly] but first and foremost health comes first, we want a fit and raring to go Gareth Bale.”
Harry Wilson could fill Bale’s free-kick role and Wilson said: “If he’s not on the pitch for one reason or another, I’ll be putting my name forward.”