The Red Devils are reportedly looking for a back-up option to Romelu Lukaku, with former Sweden global Zlatan Ibrahimovic back on the sidelines due to injury.
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Vardy is also a target but would cost United £35 million, making him the most expensive of the three attackers, per Wallace. The England forward has mustered 10 goals from 23 appearances this term and would be available to play for the Old Trafford club in the Champions League and the FA Cup.More news: £13m could bring Naby Keita to Liverpool in January
United were said to have tabled a £25m offer for Arsenal's Sanchez on Thursday but rivals Manchester City remain at the front of the queue and are confident of securing his signature this month.
The Mexico global made a name for himself as a real goal-poacher during his time at Old Trafford, particularly as a substitute, and he was hugely popular among supporters.More news: President Trump deletes tweet as Iran crisis deepens
Javier Hernandez, could find his way back at Old Trafford if West Ham can get back the £17m they spent to sign the Mexican striker last summer.
But he would want to play more than a bit-part role following a disappointing spell at West Ham under Slaven Bilic and later David Moyes.More news: CES 2018: Google announces Assistant for Smart Displays, Android Auto and more