Injured Sidibe faces season on sidelines
The Mali international, out for a long spell last season due to a knee injury, was stretchered off when he collapsed a few minutes after being introduced as a second-half substitute in Saturday's 2-1 home Premier League defeat by Tottenham Hotspur.
"It's a cruel blow for Mama because it appears he has snapped his Achilles," Pulis told the club's website.
"The medical staff have told me it doesn't look good for him although we will have to wait for confirmation from a scan he will have on the injury. If that is the damage, though, he will be lucky to play again this season, and that's a big setback for us."