The boss bemoaned the loss of Casemiro's character and influence in midfield, issues at centre-back, plus the fact that Luke Shaw and Tyrell Malacia, and also Antony and Amad, were sidelined, causing problems with both the left-back and right-wing positions.
However, there is a belief that United will benefit from more competition when the squad is back in good shape, with some of the missing Reds expected back next month.
"Sometimes, it's also some bad luck," Erik conceded, in the second part of his press conference to preview Saturday's lunchtime clash at West Ham United (12:30 GMT).