The England midfielder has joined the Hammers on loan from Manchester City until the end of the season but his mistake allowed Bournemouth to score the opening goal in Thursday's (01.02.24) Premier League clash - which ended in a 1-1 draw.
Moyes said: "It’s not a goal we concede too often and it gave them a leg up.
"Then we had to perform and we weren’t at a level to do that. It wasn’t as good as it should have been.
"Kalvin’s fine, he got the best part of 70 minutes in. I said before we might have to go a bit gently with him.
"I thought as the game went on he was starting to get better, but he was a bit slow at the start and in getting into the way we play."
James Ward-Prowse's penalty earned a point for the Hammers and Moyes was relieved to take something from a match in which his team weren't at their best.
The manager said: "I’m happy with a point but not happy with the performance. We got back in the game and I didn’t think we performed well after that either.
"It looked as if we had had 10 days off tonight, like we’d had a holiday, the players looked leggy."
Moyes was irked that West Ham had to play a match on transfer deadline day as he was unable to select midfielder Pablo Fornals after the Hammers agreed a fee for the player with Spanish club Real Betis.
He said: "It’s a strange day to have a game and also the deadline. We were planning to have Pablo involved tonight. He might still be here, who knows?
"I think it could have been wrapped up last night. It’s our own fault in this country we do so many deals in the last day. This window has been a really slow burner and we’ve had it on a game night."