Head coach Tony Pulis hailed West Brom match-winner Victor Anichebe and insisted the Baggies must end his fitness problems.
The striker struck twice in the first half as Albion beat Birmingham 2-1 at St Andrew's to reach the FA Cup fifth round.
Anichebe has battled groin problems this season and went to America for a boot camp in November to try to improve his fitness.
The former Everton striker has three goals in four games under Pulis, having previously scored just four times in his whole Baggies career.
Pulis insisted Anichebe is not 100 percent fit and believes he will be an asset once he is.
"I don't think anyone has ever questioned Victor's ability, the big thing is keeping him fit and getting him fit," he said. "He is still nowhere near and if you get him to that level I'm sure he'd be unplayable at times.
"We are coming in halfway through a season so most of the work has been done, you'd love to have him for preseason and get him fit.
"He is a good lad and is one of those if you sit down and talk to him he's responsive. We have to get him a bit fitter."
Jonathan Grounds' goal just before the break hauled Birmingham back into the game and they put the Baggies under pressure in the second half.
They rode out Blues' attacks and Pulis reiterated the need for new players before the transfer deadline.
"The Premier League is a bit different to what we faced here. We have to get some players in, there are certain weaknesses in the team," he said.
"The team is unbalanced in a way we have to sort out. We have to put certain things in place to make us a team which is going to be good in possession and out of possession. It's a massive 10 days for the club and I hope and pray things will drop for us."
Demarai Gray started on the bench after Birmingham rejected another bid from Bournemouth for the winger this week.
He tested Ben Foster twice after coming on in the second half and boss Gary Rowett explained he wanted to shield his star.
"I just thought it had been a big month for him, there's been a lot of speculation, it was a big game and I just wondered if it might be too much for an 18-year-old," he said
"I thought it would have been nice to get him out of the limelight and his pace and trickery might be best served later in the game.
"I have had lots of conversations with him recently and he has never told me he wants to leave the club so I think it's one or two people trying to stir things up.
"I hope he stays and he's best served being here until an opportunity comes his way which is befitting of his talent and it's only a top-four team who could do that."
Rowett was pleased with his side's performance, despite defeat -- just his third in 14 games in charge.
"I thought we shouldn't have been 2-0 down, that was my biggest disappointment, the way we allowed them to score the two goals. We gave them too much respect at times," he said.
"The second half was as good as we have played this season, we have pinned in and dominated a very good West Brom side."