The Republic of Ireland striker made a successful start to his career at Upton Park, scoring on his debut after securing a six-month loan deal from Tottenham last month.

But the Hammers will have to do without the 30-year-old for key games against West Brom, Stoke and Liverpool after he picked up a calf injury in last weekend's 1-0 loss to relegation rivals Birmingham.

Hammers manager Grant had hoped the striker would be fit for tomorrow's crunch six-pointer at The Hawthorns, but it is now understood that the former Inter Milan hitman will be out until at least the middle of next month.

Grant acknowledged that Keane had made a huge impression on the team since signing but is confident that the likes of Demba Ba, Freddie Sears and Carlton Cole will be able to fill the void left by the experienced striker during his absence.

"I must say in a very short time, he has had a big, big impact on the team," the Israeli said.

"He's very positive, his character, he's intelligent in his game, and he's a good player."

Grant boosted his strikeforce by signing Demba Ba on loan from Bundesliga side Hoffenheim last month.

The Senegalese hitman impressed when he replaced Keane last weekend, rattling Ben Foster's bar and causing the midlanders' defence problems throughout his 15-minute cameo.

Grant has had to contend with injuries to other key players this season and he is confident that Ba and the other strikers at his disposal will deliver if called upon at West Brom tomorrow.

Grant said: "If he (Keane) is injured, we need to play with the players we can play. We are used to this situation.

"Demba Ba is almost there. We will see how the situation is."

West Ham were celebrating today after learning they had beaten Spurs in the race to move to the Olympic Stadium.

A 13th defeat of the season tomorrow will undoubtedly quash those celebrations and shift much of the media focus back on Grant's future.

Sunday's defeat left the Hammers two points off safety, with tricky games against Chelsea, Liverpool and Tottenham on the horizon.

Grant insists he is not worried about his future and claims his side can climb the table.

"You can't tell me where there is no pressure in football," he said.

"You see the pressure at Chelsea, Man United... Everywhere there is pressure.

"In this crazy league this season, two wins takes you forward so you always have hope."

West Ham are likely to be without James Tomkins (calf) and Matthew Upson (Achilles) for tomorrow's trip to West Brom, but Grant has received some good news from the treatment room with the return of Thomas Hitzlsperger.

The German signed from Lazio on a free transfer last summer but is yet to start a game after picking up a serious thigh injury in August.

He came through a reserve game in midweek but Grant is not tempted to throw the former Aston Villa man into the team tomorrow to boost morale.

"If I wanted morale I would put the cheerleaders out," Grant said.

"The situation with Thomas is very dangerous. We want him fit, but we don't want to risk him.

"If it would be even 1% element of risk, we would not risk him."

Manuel da Costa is likely to take his place in the heart of defence tomorrow after getting through last weekend's substitute appearance unscathed.

The Portuguese picked up a serious ankle injury in October and Grant admits his return could not have come at a better time.

"In the last week we missed three or four centre-halves, so it's good that he can play," Grant said.

"He was great earlier in the season. "He didn't play in the first few games and then he was our best defender before he was injured."

Julien Faubert has travelled to the game despite a disagreement with the club last week.

The Frenchman was called upon to replace Upson on the bench after Tomkins was injured in the warm-up but he had already left the stadium after being told he had not made the match-day squad.

Faubert insists he had to go to hospital to look after his ill son but reports suggested the defender could be fined for not taking his place on the bench.

Grant refused to say whether he would discipline the player, adding: "It's an internal matter. I said to Julien what I think and the other players know that."