The American grabbed a pair of goals and was instrumental as West Ham beat Manchester United 4-0 in their League Cup quarter-final.
"I think he [Spector] is a professional guy, he was training hard when he didn’t play and wasn’t even in the squad," Grant told Sky Sports.
"We changed his position to midfield a month ago. But he got his chance tonight and he likes to speak only on the pitch, and he spoke very well today!"
The Hammers remain at the bottom of the Premier League and now with a semi-final to look forward to, Grant is looking to carry this result into the league.
He added: "It’s a new team with new players and each game we are better, and I’m very pleased with this and I hope we can continue winning.
"We have a tough game against Sunderland on Sunday but if we continue to work we’ll continue winning."
Grant was happy with the progress in the competition and knows the club must make the most of it.
He concluded: "It’s very special, to be in the semi-final is not usual for a club like us, but to perform like this against best team in the country and to score four goals. I can’t ask for more."