Manchester United's title hopes took a severe dent on Wednesday when the English Premier League champions lost 3-2 at home to Middlesbrough.
Cameroon international Joseph-Desire Job's fine and turn and shot won the game for Boro 10 minutes from time after United had battled back after trailing by two first-half goals from Brazilian Juninho.
Arsenal, who defeated Southampton 2-0 at home on Tuesday, hold a five-point lead over United with both sides having played 25 games.
Third-placed Chelsea, 2-0 winners at Portsmouth with goals from Scott Parker and Hernan Crespo, closed to one point behind United while Liverpool moved up to fourth with a 2-1 home victory over Manchester City.
Two headed goals from Juninho silenced Old Trafford and set United on their way to their second home defeat against Boro in the last three seasons.
"Boro have got a bit of a bogey on us here," Ferguson told Sky TV. "The disappointing feature was conceding the soft goals we did."
Asked how much the defeat had handed the initiative to Arsenal, Ferguson added: "We've been there before but it's not a healthy position to be in. There is plenty of time to do something about it and hopefully we can do that."
Diminutive midfielder Juninho pounced to head home from close range after 34 minutes after a shot from distance by Boro defender Franck Queudrue bounced awkwardly in front of United keeper Tim Howard, who could only palm the ball up on to the crossbar.
The Brazilian struck again four minutes later, powerfully heading home a Stewart Downing cross before Ruud van Nistelrooy gave United a lifeline right on halftime with his 22nd goal of the season.
United upped the pressure after the break and drew level on 63 minutes when Van Nistelrooy set up Ryan Giggs to stab home.
However, hopes of completing a storming comeback with a third goal were ended 10 minutes from time after Job span and fired home from 18 metres. United almost rescued a point in stoppage time but substitute Diego Forlan contrived to hit the crossbar from just four metres out.
FIRST GOAL
Chelsea's new signing Parker paid back a slice of his 10 million pounds ($18.66 million) transfer fee from Charlton last month, netting his first goal for the club with a crisp volley after 17 minutes at Fratton Park.
Claudio Ranieri's expensively assembled side never fully asserted their authority though and Patrick Berger went close to an equaliser for strugglers Portsmouth when he hit the post with a vicious long-range strike.
Chelsea soaked up the pressure and substitute Crespo, back after a calf strain, sealed the points 11 minutes from time when he followed up to chest in after Jimmy Floyd Hasselbaink's lob bounced back off the woodwork.
Ranieri was pleased with his side's performance with back-to-back FA Cup and league games against Arsenal looming.
"It was a good victory. It wasn't easy as Portsmouth at home are very strong," he said. "These three points are very important to us and now we are getting our injured players back."
Liverpool's Michael Owen ended his goal drought dating back to October 25 with the first goal at Anfield to pile more pressure on Manchester City boss Kevin Keegan.
City have not won in the league for 14 games and, despite equalising through Shaun Wright-Phillips, lost to a Steven Gerrard goal six minutes after the break.
Tottenham Hotspur were involved in another thriller, winning 4-2 at Charlton Athletic while Wales international Craig Bellamy marked his first league start since October with a goal in Newcastle United's 1-1 draw at Blackburn Rovers.
