Albion's poor form in 2020 conitnued as Stoke City inflicted the Baggies' third defeat of the season following a 1-0 win at The Hawthorns.
The Baggies missed their chance to extend their gap over second placed Leeds after Tyrese Campbell pounced on Ahmed Hegazi's error to score the game's only goal.
Slaven Bilic's side were hampered in the second minute when Grady Diagana was forced off with a reoccurance of the hamstring injury he suffered only last month.
His injury took the wind out of Albion's sails and the Baggies struggled to create any meaningful efforts to level the scores.
Matheus Pereira saw a close range header saved before he blazed over in the final moments after a half-volley from the edge of the area.
"This is no time to panic," insisted Slaven Blilic.
"A mini-crisis, yes. Hard times, yes. But we knew it was going to be no easy ride. We are still top of the league. We have to embrace the challenge and show the right character.
"We were nervy in the first half. There wasn't enough bravery. In the second half we improved but the quality wasn't there
"We had 16 shots but only four on target. You have to take more advantage of the sort of possession we had. But it's hard to create when teams are defending so deep, and left us having to rely on crosses.
"Grady Diangana's injury is another hamstring. The same leg. We will have to wait and see how he responds."