Semien, whose procedure involved repairing a fracture to the scaphoid bone, is eligible to come off the 60-day disabled list June 14, but he still has several steps ahead of him before rejoining the A's.
His latest progress was highly encouraging, nonetheless, as the A's anticipate the return of their everyday shortstop, an ironman of sorts for them the last two seasons.
"It'll take some time," Melvin said. "We're going to have to go through multiple days of [throwing], he's going to have to start swinging the bat, and at some point he's going to go out on a rehab, so we're not close as far as putting together a timetable for him, but the fact that he is playing catch is significant."
The A's have been relying on veteran utilityman Adam Rosales in Semien's stead, though Chad Pinder may have earned himself additional playing time in the middle of the diamond following an impressive defensive performance in Monday's series-opening loss at Progressive Field.
"That goes a long way for him, confidence-wise," Melvin said. "He made several good plays. It makes us more comfortable with him as well. It was tough to give Rosie a day off based on the defense, but now Pinder, that kind of day, allows us maybe to give him a day off, try to get a little offense in there with Pinder swinging the bat well."
Pinder has five home runs and 11 RBIs in 25 games since his April 16 callup.
Doolittle throws pain-free outing
Lefty reliever Sean Doolittle, on the mend from a shoulder injury, struck out four of his six batters in Tuesday's showing in extended spring camp, utilizing all of his pitches in the 19-pitch outing.
The A's will soon plot out his next step, which could be a Minor League rehab assignment.
Bassitt begins light exercises
Right-hander Chris Bassitt remains shut down from his Tommy John rehab but is engaging in light exercises, including sock throws.
Bassitt, who underwent the surgery last May, had made five rehab starts between Class A Advanced Stockton and Triple-A Nashville before expressing concerns with arm fatigue.
Jane Lee has covered the A's for MLB.com since 2010.