The Baggies were agonisingly edged out by a 95th-minute Liverpool winner, scored by Johnstone's opposite number Alisson.
“It's gutting," said Johnstone to the club's website.
“The lads gave everything. We dug in, we created chances, we made it difficult for them and we played well.
“It's frustrating to concede so late, regardless of who scores it.
“It just wasn't meant to be.
“The manager has asked for complete commitment since he came in and that was certainly on show on Sunday.
“We went out and gave it our all and we had plenty of pride.
“Even though we were relegated last week, we still came out on Sunday and we showed that we wanted to win the game and give our best.
“I'm really gutted."