Sommer said that the tie was lost when Basel allowed Chelsea to score three times in the first 15 minutes of the second half, but that reaching the semi-finals was an achievement for Swiss football.
"I'm very proud of this team," he told UEFA.com. "We have had a great campaign. It's not often that a Swiss side plays the semi-final of the Europa League – it's a big deal. We had great support today so it was a shame we couldn't get the right result.
"We said at half time that we should go out with confidence and play with good organisation but within 15 minutes they scored three times. This is too much – you can't allow that to happen against Chelsea, or the other big teams.
"Over the two games, [David Luiz] scored twice against me and the goal here tonight was a wonderful goal. There is no doubt Chelsea's individual class is very high."
Basel went ahead on the night but had no answer to Chelsea's second-half goals.