Former Manchester United defender Evans suffered a calf injury in the Baggies' 3-1 win over Hull City on 2 January, a knock that ruled him out of Saturday's FA Cup defeat to Derby County.
Tony Pulis is also hamstrung by what he called an "unbelievable situation" regarding Nyom's availability. The 28-year-old was named in Cameroon's provisional squad for the Africa Cup of Nations, despite being one of a number of players to withdraw themselves from consideration.
The former Watford defender was eventually left out of the final 23-man roster, but the club are still to receive official confirmation from the Cameroon FA over that decision, with the Baggies not ready to risk playing Nyom until they are given the all-clear.
Pulis will also make a late decision on Chris Brunt and Jonathan Leko, who both missed the defeat to Derby with knocks.
"We're a little bit disappointed Jonny's not fit, that disappoints us because we need quality in the team, especially against the top teams," Pulis told a press conference on Friday.
"We've still got that issue with Nyom as well, that's still rolling. Brunty missed the game last week so we'll have a look at Chris. But Jonny's not going to be fit and Nyom we're waiting on. The rest are getting on with it."
Meanwhile, West Brom are still looking to strengthen their first team with January additions after losing out on Morgan Schneiderlin, who completed his move from Manchester United to Everton on Thursday.
Much of the focus on West Brom's transfer dealings at the minute falls on Saido Berahino, with Stoke City manager Mark Hughes hopeful his club can clinch a deal for the 23-year-old this month.
Pulis has not given up hope of holding onto the striker though, adding there are clubs other than Stoke in the running for his signature.
"No sign of him signing a contract or going anywhere, we are in limbo. We will take it day by day," Pulis added. "It's not just Stoke involved, there are other clubs involved as well so we will see how it pans out, obviously we haven't given up hope he'll sign here."