Brett Favre says he was concerned for his good pal Toby Keith in the days leading up to the country music star's death ... telling TMZ Sports that after speaking recently with the musician, he could tell the guy was simply exhausted.
The former Green Bay Packers quarterback said he gave Keith a ring just two weeks ago to check in on him ... after he noticed the 62-year-old didn't look so hot following some December performances in Las Vegas.
Favre -- who told us he would call Keith every three-or-so months following his stomach cancer diagnosis -- said the "Red Solo Cup" crooner complained about the damage chemotherapy had done to his body during .