That's a pretty simple lesson and one that many teams have learned in the past, so it's very surprising that Spurs boss Mauricio Pochettino allowed his players to grant so much green grass - well, actually brown dust considering the shocking state of the Wembley "turf" - to the visitors. He held the ball up, tracked back and ran around like a man possessed in the second half to try and get his team back in the game.
There is no doubt that Barcelona are prioritising the Champions League this season and the club and Lionel Messi have made that clear but the Argentine insists they will not be holding back in any competition.
In recent weeks there have been murmurs that Barcelona have been struggling after three games without a win in La Liga. He played a massive part in two of the four goals while scoring the other two.
By the time Jordi Alba virtually cut the ball back from the byline, Messi had darted into the box and both Coutinho and Suarez left it for the five-time world player of the year to finish. The Catalan side doubled their lead in the 28th minute, Rakitic firing a superb volley in off the post after good work from Messi and Coutinho down Spurs' right.
Tottenham's fatigue is largely down to the World Cup exertions of many of Pochettino's players, but also a result of the lack of new additions during the summer.
"The Champions League is the icing on the cake for everyone", he said after the game.
The main area Barcelona can improve is defensively. Samuel Umtiti saw red against PSV and won't be eligible.
"But for me the most important thing was that in the second half we played and we showed character".
And when Messi made it 4-2 in the 90th minute, it was Suarez once again whose dummy created space for Barcelona's No.10 to score. It will be tough because they are must-win games.
Pochettino has a host of injury problems to cope with, as Christian Eriksen, Dele Alli, Jan Vertonghen, Serge Aurier and Mousa Dembele are all out. Heung-Min Son, Lucas Moura and Erik Lamela were all looking for the ball in-behind and nobody was there to slot them in, even though Harry Winks put in a fine display to try and shut Barcelona down.