The news of Morrison's injury comes as a major blow to Pulis, who is also set to be without creative winger Matt Phillips once again. The Scotland international has missed the Baggies' last three matches with a hamstring problem and looks set to miss the clash with the Gunners.
"Unfortunately Morrison has hurt his ankle this week so we're not sure whether he's going to be fit," Pulis said in his press conference. "We've missed Matty, and it's been a big miss. It's looking at the depth of the squad. It's a real test for all the players who are available to get the points that we want."
West Brom have enjoyed a largely successful season so far and are currently eighth in the Premier League, though they have lost their last two matches in rather disappointing fashion. Despite their troubles over the last few weeks, Pulis thinks his players have done 'fantastically well' in recent months and will hope they can get back to winning ways against Arsenal, who have lost three of the four last league matches.
"I think the players have done fantastically well," said Pulis. "When we put our top team out the balance of the team is good. You need a balance to win games as well as anything else. With Phillips in the team there's that balance and that's a player we do miss. That's a big thing."
Pulis' side may have taken their foot off the gas since reaching the glorified 40-point mark but will be comforted by the fact that they defeated Arsenal in the same fixture last season.
The Baggies almost secured a positive result at The Emirates Stadium earlier in the campaign but their attempts to leave north London with a point were dashed four minutes from time by Olivier Giroud, whose late header handed Arsenal a precious victory.