The game was a classic in front of the BT Sport cameras as the ten man Ammies came back twice but suffered heartbreak in the final seconds.
Salford lost the game 3-2 but Alexander says they even in defeat they proved they have promotion fight in them.
"The game changed when the first big decision was made," Alexander told the club's video service.
"It was the wrong decision, it hit his chest and it was the first time they had been in our box.
"Of course we're disappointed but what I will say about my team is how superb they were in the second half.
"We said at half-time that he had an opportunity to make sure we didn't have criticism coming our way and I think we did that.
"I thought we showed the best of ourselves in the second half, even though it didn't end as we wanted it to.
"We scored two fantastic goals with ten men which should give us at least a point but we have been undone by a late header four yards out. We certainly have to defend better."