former和was/is的使用搭配疑问？

Mike is a teacher now, but he is/was former worker.

要说他以前是工人，如果有了“former以前的”，是否可以不用was，而用is呢？ 

如果没有former,当然要用was.

但是因为已经有了former表明是以前，还需要用was吗？是否可以用is呢？

美国之音原句：

Evo Dalder is a former U.S. ambassador to NATO.

He has made it clear frequently that he not only thinks

that Europeans aren't doing enough, but that......

上句用了Evo Dalder is a former U.S. ambassador to NATO.

没有用was.

因此，Mike is a teacher now, but he (=Mike) is???/was??? former worker.

应该是is,还是was,还是二者均可？？？