The Bees had a first minute lead through Bryan Mbeumo before Tomas Soucek struck in the second-half.
Lopetegui said, "We knew that we had a very tough match, they won both matches here against Southampton and Crystal Palace and we suffered by conceding in the first minute.
"We showed this (Brentford's record of scoring early) to the players and it's incredible we suffered. It's a bad picture to concede so early.
"We kept calm and I think we deserved to win the match in the second half, we showed character and commitment.
"We had three or four very clear situations to score a second goal but we didn't manage it and we take a point. I have to keep positive.
"We always have to work as a team or we are weaker - I think we did this in the second half."