The 31-year-old captain, who has spent his entire career at the Basque club, did not seek to soften the message or deflect scrutiny from himself or the squad.
"This has not been an easy season. Not personally and not as a team. As captain I take my share of responsibility. The team has suffered a great deal this season."
He acknowledged the frustration felt by supporters and was explicit that criticism is legitimate when results fall short.
"We fully understand the discontent. Athletic always demands giving everything, and people are free to express their opinion when things don't go right."
Williams defended the commitment of the dressing room while accepting that effort alone does not always translate into points.
"The group has not stopped working for a single day, putting all its heart into it. But in football, as in life, sometimes effort does not translate into the results you desire."
Athletic are still in contention for a Europa League or Conference League place heading into the final two fixtures. The club are fighting on the fringes of the European spots, and Williams was clear that the season is not yet decided.
"Even so, this is not over. There are two matches left and we still have the chance to fight for Europe. We will leave everything on the pitch for this badge and for everyone who is always there."
Athletic Club host Celta Vigo on Sunday before a final-day fixture that could prove decisive. A European place would represent a significant recovery in a season that had threatened to end with little to show for a squad widely expected to compete at a higher level.