Spurs were beaten 1-0 by the visitors, who went into the game bottom of the Premier League, just three days after travelling to Belgrade to take on Partizan in European competition.
Pochettino's side drew that game 0-0 but it was a much-changed line-up that succumbed to a 1-0 loss at White Hart Lane, with James Morrison scoring a 74th-minute winner.
And the Argentine refused to use Tottenham's midweek exertions as an excuse for the result.
"It's not an excuse," said Pochettino.
"We played with 10 different players from the game against Partizan in Belgrade on Thursday. I believe that we were in a good condition to play the game today and to have a good game with a good performance but it is strange that we played very slow."
He added: "I think the beginning of the game was wrong. We were very slow, we moved the ball very slowly, and we had a very slow tempo.
"In football you need to play quickly because the opponents sometimes, like West Brom today, play a bit deeper. You have to try to recover the ball and be aggressive in offensive situations."
Pochettino usually cuts a calm figure on the touchline but appeared animated and frustrated by Spurs' poor execution.
He said: "I don't know what is the best word to show my feeling but I am very disappointed because we had a very bad day, we played very slow and, at times during the game, the team looked very nervous.
"I think the result it not fair because I think the draw is fair when you analyse the game.
"But in the end if you make a mistake, you lose and this is why we have zero points."
Spurs could muster only one shot on target as they were beaten at home by West Brom for the first time in 30 years and are now without a league win since August 24.
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