The Heater says there is "reason to believe both will join the team today."
Hernandez is essentially a life long minor leaguer. He recieved a few at-bats with the Yankees years ago. He's a line drive and groundball hitter although I wouldn't expect much from him offensively. In Indianapolis Hernandez threw out 19 of 74 attempted base stealers or 20%, John Jaso's numbers for comparison are unavailable.
Johnson is a former top prospect with the Oakland Athletics who the Rays claimed on waivers way back in May. He was promptly designed for assignment a day later and after clearing waivers has killed Triple-A pitching. Johnson has walked and struck out at nearly a 1:1 rate in the majors. He bats lefty and is not a pretty sight in the field, but his bat is good.
And Joel Guzman has been designated for assignment. I'm not sure if someone will claim him on waivers. Johnson apparently might start in left.