Parma beat Milan at San Siro for the first time this century as ex-Milan forward Antonio Cassano scored twice in a 4-2 victory.
Milan – who were knocked out of the UEFA Champions League's last 16 on Tuesday – were hampered from the fifth minute after goalkeeper Christian Abbiati was sent off for bringing down Ezequiel Schelotto in the penalty area.
Cassano scored the resultant penalty and doubled his side's lead six minutes after the break, only for 10-man Milan to respond with an Adil Rami header and Mario Balotelli's spot-kick.
After Milan's pre-match was marred by fan protests against vice-president Adriano Galliani and Balotelli, they slumped to defeat though – late goals to Amauri and Jonathan Biabiany sealing Parma's triumph.
"The players gave all they had today, trying to get back a game that started in the worst possible way," Seedorf told Sky Sport Italia.
"It was a strange game and obviously the red card after five minutes changed everything.
"The lads did well to fight back to 2-2, but their determination to get the victory ended up leaving the team unbalanced and we suffered this defeat.
"We never gave up. At half-time I told the players to believe, as even with 10 men we've seen before that teams can improve and get important results.
"They did well to get back on the field with that reaction and on 2-2 we perhaps should've read the situation better to avoid the counter-attack."
Seedorf also revealed that he, and a group of players including Balotelli and Kaka, went to meet the club's ultras after the match and they had a "peaceful" meeting.
Parma boss Roberto Donadoni – who won six Serie A titles and three European Cups in two spells as a player at Milan – hailed the attitude of his players, despite saying that they could play better.
"I want to be honest, it wasn't our best performance," Donadoni said.
"I have to praise my players, as it was not easy to win at San Siro, especially with our opponents in a curious psychological situation.
"There enthusiasm is remarkable. We went 2-0 up, Milan caught us, but then we scored another two.
"I like that spirit. Of course there can be errors along the way and things to be improved, but that's what this job is all about."
Cassano feels his team are playing in an "extraordinary" fashion and expects their unbeaten record to continue.
"It's a wonderful moment, but the team is doing something extraordinary," Cassano said.
"I had great moments at Sampdoria but they are equally wonderful here.
"We're unbeaten in 16 and I think we can do even better."
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