Sir Alex Ferguson sent out a relatively strong side and was rewarded as Cristiano Ronaldo, made his first start of the season after an ankle surgery, heading home the opener after 25 minutes.
Substitute Adam Johnson snatched an equaliser for the visitors in the 56th minute, volleying home a 25-yard effort past fourth-choice keeper Ben Amos.
Then there was a reckless challenge from Boro captain Emanuel Pogatetz on Rodrigo Possebon which saw the 19-year-old Brazilian require lengthy treatment before going off on a stretcher as the game was held up for seven minutes.
Possebon, the brightest of a gifted crop of Red Devils' young guns, went to control a loose ball only for Pogatetz to fly in knee-high and send the player spinning in the air.
Pogatetz was sent off and United made 10-man Boro pay in the 79th minute when Wes Brown played a deep ball from the back into the path of Ryan Giggs who took it on and neatly chipped home.
Then in stoppage time, Nani added a third after a miss backpass from Boro defender David Wheater.
At Emirates Stadium, Arsenal boss Arsène Wenger stuck to his policy of giving youth a chance in a domestic knock-out competition.
No less than seven of Gunners' starting line-up had yet to celebrate their 20th birthday with the average age of this team was just nineteen!
And these young Gunners made a remarkable night as they romped Sheffield United 6-0.
Nicklas Bendtner - at 20 one of the senior men on the field and with a wealth of first-team experience already - scored Arsenal's opening two goals before Carlos Vela registered his first before half-time.
The young Mexican scored his second five minutes after the restart with a delightful chip.
It got worse for the visitors when Jack Wilshere - youngest Gunner to appear in the Premier League - drilled in a low shot from 22 yards on 57 minutes.
Vela then completed his hat-trick with another well-taken effort after 88 minutes.
Liverpool manager Rafa Benítez changed his entire line-up for the third round match of the Carling Cup against Crewe Alexandra.
And it nearly left him with a big embarrassment.
Daniel Agger thundered home a free-kick to give his side a 15th minute lead before Crewe equalised 10 minutes later through Michael O'Connor.
But Liverpool restored their lead in the 58th minute when Lucas headed home Jermaine Pennant’s cross from the right flank.
Rafa surprisingly introduced Fernando Torres to give the Spaniard a chance to score after goalless since the opening day at Sunderland.
But the hitman couldn’t find the net in a 25-minute run-out.
Substitute Adam Johnson snatched an equaliser for the visitors in the 56th minute, volleying home a 25-yard effort past fourth-choice keeper Ben Amos.
Then there was a reckless challenge from Boro captain Emanuel Pogatetz on Rodrigo Possebon which saw the 19-year-old Brazilian require lengthy treatment before going off on a stretcher as the game was held up for seven minutes.
Possebon, the brightest of a gifted crop of Red Devils' young guns, went to control a loose ball only for Pogatetz to fly in knee-high and send the player spinning in the air.
Pogatetz was sent off and United made 10-man Boro pay in the 79th minute when Wes Brown played a deep ball from the back into the path of Ryan Giggs who took it on and neatly chipped home.
Then in stoppage time, Nani added a third after a miss backpass from Boro defender David Wheater.
At Emirates Stadium, Arsenal boss Arsène Wenger stuck to his policy of giving youth a chance in a domestic knock-out competition.
No less than seven of Gunners' starting line-up had yet to celebrate their 20th birthday with the average age of this team was just nineteen!
And these young Gunners made a remarkable night as they romped Sheffield United 6-0.
Nicklas Bendtner - at 20 one of the senior men on the field and with a wealth of first-team experience already - scored Arsenal's opening two goals before Carlos Vela registered his first before half-time.
The young Mexican scored his second five minutes after the restart with a delightful chip.
It got worse for the visitors when Jack Wilshere - youngest Gunner to appear in the Premier League - drilled in a low shot from 22 yards on 57 minutes.
Vela then completed his hat-trick with another well-taken effort after 88 minutes.
Liverpool manager Rafa Benítez changed his entire line-up for the third round match of the Carling Cup against Crewe Alexandra.
And it nearly left him with a big embarrassment.
Daniel Agger thundered home a free-kick to give his side a 15th minute lead before Crewe equalised 10 minutes later through Michael O'Connor.
But Liverpool restored their lead in the 58th minute when Lucas headed home Jermaine Pennant’s cross from the right flank.
Rafa surprisingly introduced Fernando Torres to give the Spaniard a chance to score after goalless since the opening day at Sunderland.
But the hitman couldn’t find the net in a 25-minute run-out.
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