Caldwell suffered a fractured cheekbone even though he bravely completed the full game as the Latics had already used their full compliment of substitutes.
A late goal from Albion substitute Marc-Antoine Fortune salvaged a share of the spoils with Wigan seemingly heading for victory.
Mixed feelings: Wigan boss Roberto Martinez was pleased with the result but not with outcome
Martinez said: 'Gary Caldwell had a fractured cheekbone and that is the biggest negative we picked up. He is a true captain and a great leader and we are going to miss him. We are going to assess him in the next 24 hours but it is not really good news.'
Caldwell's injury took some of the gloss off Martinez's joy at hanging onto dangerman Charles N'Zogbia for the remainder of the season.
Charlie boy: Charles N'Zogbia undelined his importance to Wigan with the equaliser
N'Zogbia is believed to have the been focus of a last-ditch attempt by Newcastle to sign their former player before the transfer window shut. But N'Zogbia has remained with the Latics and played a starring role with his side's opening goal.
Martinez said: 'Our intention this transfer window was to finish with a stronger squad. We had two major objectives, getting all the players back from injury, and we didn't want to lose any of the prize assets.
'We managed to do that. In the past, the fans have been a bit disappointed at losing players at the wrong time.
'Everyone is proud that Wigan showed a bit of a statement by keeping the players for the important challenges we have at the moment.'
Baggies boss Roberto Di Matteo felt Albion should have been awarded a penalty late in the game after Gary Caldwell appeared to handle the ball.
He said: 'We didn't get a penalty again. It seems like every game now. We didn't get one in this game, against Blackburn, against Manchester United. They are adding up and they are costing us as well.'
Di Matteo was pleased with the debut of on loan Arsenal player Carlos Vela who played for just under an hour.
Top Gunner: Carlos Vela impressed his new manager on his debut
He said: 'I think he had a good start to the game, was dangerous and he will be a very good addition for us.'
Peter Odemwingie put Albion ahead but N'Zogbia and Ben Watson struck back before half-time only for Fortune to salvage a point for the home side.
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Explore more:People: Marc Antoine Fortune, Ben Watson, Gary Caldwell, Carlos Vela, Roberto Di Matteo, Roberto Martinez Places: Newcastle