Assuming the bits are actually flesh and blood, the main question is whether or not they used to be bread and wine.
The only evidence for this transformation occurring is the long-ago testimony of a monk sometime near the year 700:
During the 700th year of Our Lord [...] a monk of the Order of St. Basil was celebrating Holy Mass according to the Latin Rite. Although his name is unknown, it is reported in an ancient document that he was "... versed in the sciences of the world, but ignorant in that of God." (Cruz 1984, at p. 10)
With this being so long ago, and very little documentation surrounding the incident, I would treat descriptions of what happened as hearsay.
Philosopher David Hume explained:
No testimony is sufficient to establish a miracle, unless the testimony be of such a kind, that its falsehood would be more miraculous than the fact which it endeavors to establish.
In this case the testimony's hearsay nature makes the likelihood of its falsehood quite high.
Meanwhile, science has a very solid understanding of matter and chemistry. Under our current understanding, such a transformation is not possible. The likelihood of that being so substantially incorrect that bread could transform to flesh and blood in the hands of a doubting priest is very, very low.
This hearsay account is not enough to be persuasive that a miracle occurred.
Joan Carroll Cruz. Relics. Our Sunday Visitor Publishing, 1984.