"Time to take the training wheels off," manager Bob Melvin said.
• A's Spring Training info
Henderson underwent shoulder surgery in July 2015, signed a one-year deal with the A's in December and is expected to join their rotation in May. Simply the thought of this enlivens Melvin, who could soon have a solid 1-2 rotation punch in Sonny Gray and Alvarez.
"Like an in-season free-agent signing, a key one," Melvin said. "He's going to be really good for us barring any setbacks. He's got quite the track record.
"He's got a nice clean delivery, but he's got a quick arm. It's the kind of guy that beats the guy to the catcher's glove, a guy that, if he's throwing 92, it seems to play a little faster with some late giddy-up at the end."
Alvarez, who turns 26 next month, struggled with the Marlins ahead of his surgery last year, but he was outstanding in a 2014 All-Star season, totaling 187 innings with a 2.65 ERA.
Alvarez's looming arrival could make for some interesting rotation decisions and foster in-season competition among several of the current projected starters behind ace Gray: Rich Hill, Jesse Hahn, Kendall Graveman and Chris Bassitt.
"I think they're very much aware of that," Melvin said.
Outfielder Sam Fuld, sidelined by a shoulder sprain, was able to be at home in Florida with his wife, Sarah, for the birth of his fourth child on Thursday. John Spencer Fuld weighed in at 9 pounds, 1 ounce.
Jane Lee is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.