Ronny Deila says his team's efforts in pushing Inter Milan to the limit in their Europa League tie demonstrates how far the side have come this season.
The Scottish champions succumbed to a 4-3 aggregate defeat after Fredy Guarín's late strike consigned them to a 1-0 second-leg loss in the San Siro.
"You ask me a lot of times about progress; we get the answers in Europe," the Celtic manager said.
"I think we showed we've really, really improved. This is just the start."
Celtic held on resolutely in Milan after Virgil van Dijk's sending off in the 36th minute left them down to 10 men, before Guarín's 88th minute stunner put the tie out of reach.
Deila says the performances over the two legs show his side can compete in Europe, and that they are only going to get better.
"These players are young and it's just the start of their careers for a lot of them," Deila said.
"I'm very optimistic for the future. Everybody is starting to understand what it means to be 100% every day, to put effort every day in training and be prepared, and be 24-hour athletes. That's why we get better.
"You see now the level in running and sprinting and fitness, it's a different level now than we were in the summer."
Celtic face Aberdeen in a top-of-the-table clash at Parkhead on Sunday, and Deila says it is the biggest match since he arrived at Celtic Park as manager.
"In my opinion this is the most important game we have so far this season," the Norwegian added.
"The league is everything and if we win this game it's going to be very hard to reach us again.
"The pressure is of course on both teams, but Aberdeen has to win, maybe a draw, to stay in the fight for the championship.
"This is a very, very important game for us."
Deila feels Aberdeen's challenge for the title is a good thing for Scottish football, but would prefer his own side had a greater cushion at the top of the Premiership.
"For the whole country it's important to have excitement in the league, but I'd rather stay top of the league alone and be comfortable," he said.
"But Aberdeen has deserved it. They've played very well and it's going to be a big game on Sunday."