Melvin hopes to have Treinen at the ready come Friday in Houston, following Thursday's off-day.
"It was a little disconcerting," A's starter Andrew Triggs said, "so hopefully it's just a bad bruise and he'll be fine."
The liner, hit off the bat of Shin-Soo Choo, had an exit velocity of 105 mph, per Statcast™. Choo reached first base for a single, and Treinen was quickly escorted off the field with a trainer, though he was walking under his own power.
Right-hander Lou Trivino, recalled from Triple-A ahead of the game, replaced Treinen, teaming up with Santiago Casilla to preserve the lead.
Jane Lee has covered the A's for MLB.com since 2010.