#1 Zlatan Ibrahimović

This might be a bit of a shocker, but hear me out here. Zlatan Ibrahimović was phenomenal in the first stint with United. For a player his age, to come to Premier League and score as many goals as he did, without playing the whole of the season, is just phenomenal.
But, then again this is Zlatan Ibrahimović we are talking about. All was going well until one Europa League night in April, where both Marcos Rojo and Zlatan Ibrahimović suffered ACL injuries. Resigning Zlatan Ibrahimović after the injury was a poor and purely emotional decision from Mourinho and was very unlike him.
28 goals in the first stint and just 7 in the second prove that he was rushed into playing and was a poor signing on Mourinho's part. He suffered a further injury for having been rushed and that injury kept him out till December. in the subsequent month, he moved to MLS.