Jurgen Klopp said there was "no chance" James Milner was not going to be brought on for his 600th Premier League appearance on Saturday.
Milner came on as a 68th-minute substitute during Liverpool's 3-1 win over Southampton at Anfield.
That saw the 36-year-old become the fourth player in Premier League history, after Gareth Barry, Ryan Giggs and Frank Lampard, to make 600 appearances in the competition.
Klopp, who was watching from the stands as he served a one-match touchline suspension handed to him by the Football Association (FA) on Friday, knew Milner was closing in on the milestone, and said there was little chance the former Manchester City and Aston Villa midfielder would not play.
"Wow! I don't know how many have reached that mark in England. Most of the games probably for Liverpool so it's an honour to be around when he reaches that number. Even Millie is proud today," Klopp told reporters.
"It's a big number, a big career and won pretty much everything. A role model for us, for everyone in the room. I knew he was on 599 and there was no chance he wouldn't play today.
"He did well, you could see that he is still important to us. He doesn't just play to get him number 600.
"He sets the tone very often, has the right amount of aggression. He knows that at 36, he has room for improvement, it's crazy. But I'm glad to have him. Congratulations James."
Liverpool have won 10 of their last 11 league meetings with Southampton, with Darwin Nunez's double seeing off the Saints, who were playing for the first time under new manager Nathan Jones.
Roberto Firmino's opener – his 18th headed goal in the Premier League – had been cancelled out by Che Adams, whose four top-flight goals this season have come away from home.
The victory sees Liverpool go into the break on 22 points, having won their last two league games following successive defeats to Nottingham Forest and Leeds United.
"Very, very pleased," said Klopp. "Three points was the most important thing. The performance in the first half was really good as well.
"Early goal, early goal then for them but a good reaction. We got in behind and used our speed, used our football skills.
"Second half wasn't as good. We needed Alisson. He enjoys that but as a manager I enjoy it much less. We didn't control the game well enough in the second half. Defensively we were too passive, too deep and that's how they were able to get through.
"But 22 points was the maximum we could reach before the break, we knew that for a little while and that's what we've got."