A 3-1 loss at West Ham on Saturday left United with 10 points from seven games — matching the club's previous worst Premier League start under Moyes in 2013 immediately after Alex Ferguson's retirement.
United are top of Group H after beating Young Boys in their opening match but have not won any of their three matches since in all competitions.
Manager Jose Mourinho said some of his players "care more than others" in his pre-match news conference when asked about the winless run.
The match is the first of two consecutive Champions League home games for United with Juventus visiting Old Trafford on 23 October.
Spanish side Valencia, competing in the Champions League for the first time in three seasons, are third in the group after losing 2-0 at home against Juventus in their opening fixture.
They are 14th in La Liga with just one win in their opening seven games but have only lost once.
Manchester United: De Gea, Valencia, Smalling, Bailly, Shaw, Fellaini, Matic, Pogba, Rashford, Sanchez, Lukaku.
Valencia CF: Neto, Piccini, Garay, Paulista, Gayà, Coquelin, Parejo, Kondogbia, Guedes, Rodrigo, Batshuayi.