Antonio Conte's men welcome the Blaugrana to Stamford Bridge for their Champions League last-16 first leg tie. The Catalan giants are cruising in La Liga and are coming off a 2-0 win over Eibar on Saturday.
And Alonso understands he and his teammates must be at their very best to advance to the quarter finals.
"We know they've been one of the best teams in the world in recent years, if not the best, and it will undoubtedly be a very tough tie," he said.
"I think we'll have to defend very well and take our chances because, like I said, they're a very tough team to beat, let alone over two legs. Still, we will prepare as best we can and give it a shot, and we'll see how the tie unfolds.
"The general consensus has been that it's better to be at home in the second leg, but I don't think it matters much.
"Obviously, it will be important to acquit ourselves well on home turf in the first leg so that we're in with a good chance going to the Camp Nou, which is a very difficult place to win."