Former Liverpool defender Jamie Carragher was surprised by the "big price" Manchester United paid for teenage forward Anthony Martial.

The 19-year-old Frenchman completed a 36 million pound ($55 million) move from Monaco on Tuesday in what was the highest transfer fee paid for a teenager.

Carragher, 37, said it was a huge price to pay for an unproven forward, who scored 11 goals in 52 Ligue 1 matches for Lyon and Monaco, as United look to replace the likes of Robin van Persie, Radamel Falcao and Javier Hernandez.

"I think so," Carragher said, when asked if he was surprised by the price."He's the most expensive teenager in world football, so it's a big price. He comes with a lot of potential, which is expected at that age, just getting to the French squad as well."

Carragher said the opportunity was right for Martial to move, but the price tag will carry plenty of expectations. 

"With Monaco going out of the Champions League [in the play-offs] as well, it made it easier for Manchester United to get him," he said. "But it's certainly a position they needed to strengthen, striker-wise, with the players that have moved on.

"He's a young player and when you talk about potential you're still not quite sure but for that price, he must be some player."

Carragher said he was looking forward to the focus turning to on-field matters after the transfer window "madness".

He believes most managers would welcome the window closing earlier.

"There's a lot of excitement, last-minute deals getting done," Carragher said."You always wonder why it takes so long to get the deals done. You look at the Manchester United-Real Madrid one [with David de Gea] as a case in point really, why you have all summer and it goes to the wire and it doesn't quite get done.

"It's exciting for the fans. I don't think managers like it. I think they want to call it to a halt once the football starts, but I think for the man on the street it's quite exciting."

He added: "It's done now; get on with the real stuff - the football."