West Ham manager Avram Grant praised the impact of Thomas Hitzlsperger after the German midfielder helped the Hammers to a resounding 3-0 victory against Stoke on Saturday.
The result sealed West Ham's first back-to-back league wins for more than a year and significantly boosted their hopes of escaping relegation from the Barclays Premier League come the end of the season.
The Hammers' resurgence has coincided with the return to fitness of summer signing Hitzlsperger, who rattled in his second goal in three games following his recovery from the injury that prevented him from playing for his new club until last month.
Three and easy: Thomas Hitzlsperger celebrates with Frederic Piquionne after scoring West Ham's third goal against Stoke
Grant, who brought in four players in the January transfer window, said: 'Thomas is like a fifth signing in January.
'He's had a big impact because, with him, our midfield has more balance. He knows how to defend, he can pass the ball, he can score.
'He's important for us but I must say all the new signings are doing well.'
Demba Ba opened the scoring against the Potters before Manuel da Costa headed a second and Hitzlsperger wrapped up the points late on.
Birmingham boss Alex McLeish felt his side suffered a hangover from their Carling Cup triumph over Arsenal as they slipped into the relegation zone with a 3-1 defeat against local rivals West Brom.
Goals from Youssouf Mulumbu, James Morrison and Paul Scharner sealed victory for the Baggies, who have moved above their opponents in the battle for survival.
All that Birmingham could manage in reply was a goal from winger Jean Beausejour, and McLeish was left to reflect on what might have been after having to make a number of changes to his side.
McLeish said: 'Wembley was a factor in our performance. To make the enforced changes we had to do made it very difficult for us.
'You are looking for the guys who come in to provide fresh inspiration, but it did not materialise, and the goals we lost were schoolboy errors.
'It is a kick in the teeth, and we know the battle we face.'
Eye for goal: Hitzlsperger scores a third goal for the Hammers at Upton Park
Sunderland manager Steve Bruce was pleased with the performance of his players as the Black Cats weathered the storm to secure a goalless draw with Arsenal at Emirates Stadium.
Arsenal's Andrey Arshavin should have been given a penalty after being pushed by Titus Bramble as he shaped to shoot, and then had a goal incorrectly ruled out for offside, but Bruce felt his side deserved their luck after the 2-0 loss to Everton in their previous outing.
'We needed a response after last week when we were shocking,' he said. 'We have been hit by injuries, but the response today was terrific.
'There were a few hairy moments in the second half, but overall I think we deserved something from it.'
Arsenal's slip saw Manchester City close the gap on the second-placed Gunners to four points as a David Silva strike lifted Roberto Mancini's men to a 1-0 win over Wigan.
Gary Cahill scored twice and Ivan Klasnic bagged the winner in Bolton's thrilling 3-2 win over Aston Villa, while Fulham also won 3-2 against Blackburn, Bobby Zamora's penalty sealing victory for Mark Hughes' men in the final minute.
At St James' Park, Everton fought back from a goal down against Newcastle to post a 2-1 win. Goals from Leon Osman and Phil Jagielka secured the points after Leon Best had opened the scoring for the Magpies.
Hammers on the rise after another win over StokeGrant slates rivals over approach for hot-shot Ba
Explore more:People: Ivan Klasnic, Steve Bruce, Mark Hughes, Phil Jagielka, Paul Scharner, Gary Cahill, Leon Osman, Leon Best, Andrey Arshavin, Thomas Hitzlsperger, Alex McLeish, Titus Bramble, Roberto Mancini, Bobby Zamora, James Morrison Places: Newcastle, Birmingham