Bale's calf injury is no joke. The Welshman and Real Madrid's medical staff thought that he could be recovered in time for Real Madrid's first leg of the Champions League Round of 16 tie against Roma but he hasn't made a lot of progress and it looks like he will have to sit that one out, according to Marca.
Bale had been working hard to get ready for that game but he suffered a relapse on that calf, per that same report. The Welsh attacker has had problems with that calf ever since he joined Real Madrid, so it's a tricky injury that will need some time to heal properly.
Zidane and the rest of the coaching staff will surely need Bale's attacking presence down the road, so it seems like a good decision to wait and let that calf recover the way it should so that he can play more often.