It now appears the Old Trafford club are chasing Dynamo Moscow's Aleksandr Kokorin as another alternative, as reported by Russian news agency TASS.
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According to the agency owned by the Russian government, Kokorin's representatives are in talks with officials from Man United and Tottenham Hotspur over a potential move.
The 24-year-old has scored 45 goals in 193 games for Dynamo Moscow, along with eight in 32 appearances for Russia, including a goal against Algeria at the 2014 World Cup.
UNITED MOVE ON FROM PEDRO
Manchester United fans were severely disappointed having lost out to Chelsea in the race for Pedro, but may be buoyed by a potential triple swoop before the window closes.
The Red Devils are chasing World Cup winner Muller, along with Mane and Kokorin.
The Saints forward, while not on the scale of Pedro, would be a good signing for United.
He scored ten goals in his debut season at St Mary's, including a Premier League record fastest hat-trick of two minutes 56 seconds.
At 23, time is on his side and he has the potential to become one of the top forwards in the English top flight, however he will not come cheap for United.
Kokorin is an unproven entity outside of Russia, but has built up quite a reputation in his home nation as one of their brightest talents.