Since Ander joined, he has started 91 PL games. We won 46 drawn 25 lost 20. During this period we have played 92 PL matches when Herrera didn't start. Our record when he doesn't start is 54 wins 22 draws and 16 defeats. If you then adjust for changes after he was substituted or came on as a sub, the net effect is zero, with 11 points gained being cancelled out by 11 points lost (If Herrera comes on as a sub with the score level then we win the game, that's an improvement of 2 points. Alternatively, if he started then was substituted with the score level but we win the game, that's a loss of 2).
Herrera started 42 home and 49 away, which goes some way towards explaining the gap, as does starting slightly more matches against the big 5 than he didn't start. On a like for like basis, taking into consideration the strength of opponent and home/away, our record with him playing is very similar to when he hasn't been on the pitch.
I like Ander and until recently he has performed very well under Solskjaer but the club needs to be a lot more careful about the type of contracts it offers to players in their 30s (or in this instance, will turn 30 before the start of the new season), especially those in positions in which work-rate is vital. We have a long history of ridiculous contracts given to Rooney, Schweinsteiger, Sanchez etc when a decline in performance is exactly what you'd expect given the age curve of most footballers.