The Serie A outfit had gone in front at the Commerzbank-Arena through Ruben Botta, but on-loan Chelsea man Lucas Piazon levelled for Frankfurt.
Haris Seferovic then netted twice before half-time as Inter succumbed to defeat in their penultimate friendly before their return to competitive action in the UEFA Europa League play-offs in 10 days' time.
Mazzarri, while disappointed with the result, felt the hard work put in by his players during the week had been detrimental to their short-term cause.
"Was this down to the heavy workload during the week? I don't want any excuses," he told Inter Channel.
"My fitness coach told me we needed to work on strength right now, as otherwise we'd lack sharpness later on in the season. I think the lads paid that price and they looked pretty heavy-legged.
"We weren't sharp and I hope that is only down to the fitness work. At this stage of the season it's easy to do well in one game and badly the next. If you lack just a tiny bit of sharpness and focus, then you can trip up.
"Up until our goal Eintracht hadn't created any chances, but we have to be more concentrated in some situations. Perhaps we subconsciously let go after taking the lead.
"In the second half we focused on damage limitation."
