Guimaraes struck the opening goal on 71 minutes, but when leading 2-1 in injury-time, Cristian Romero struck a spectacular 95th equaliser.
Guimaraes told Sky Sports: "It's tough to say about the game. We are leaving here today very unhappy.
"The ball was bouncing in the box and nobody was there to take the ball away including myself. The feeling right now is very negative, but we have to turn things around. It is unacceptable for us to concede again at the end of the game."
On scoring his goal, he went on: "It was a brilliant finish, but I would change everything about my goal to get the three points. I can't leave happy today, but we have another game Saturday, so we have to take the positive and try to get the three points in that match.
"It's not the first time we concede at the end of the game, it's something we have to sort out.
"It's not healthy for us to concede so late. In my view we were better than them, so we leave unhappy tonight."