Kobe Bryant IN the Gym with Manny Pacquiao

Kobe Bryant is back in the gym -- not with the Lakers (stupid lockout) -- but with Manny Pacquiao!

Kobe slipped unnoticed into Manny's LA workout spot ... Wild Card Boxing Club on Thursday afternoon -- and just over an hour later, we spotted him leaving through the back door.

We're told Manny was definitely working out while Kobe was there -- people outside heard Manny punching the crap out of stuff -- but we're pretty sure Kobe was not IN the ring ... Black Mamba was unscathed when he left.

Would be a helluva fight though.

Source: TMZ