The 28-year-old had travelled to London and agreed personal terms but the club were forced to call off the transfer late in the day.
Says Glenn: "It's very disappointing that the deal has fallen through. There was no problem with the financial part of the deal and the player had agreed terms but other aspects made it impossible for us to complete the transfer.
"I am also disappointed that Fiorentina released a story claiming that Pierini had signed for us - before he had undergone a medical and before any deal had been signed.
"That gave the wrong information to our fans and, with there always being a chance that a possible deal can fall apart before it is concluded, it wasn't fair that they should be misled like that.
"I understand that the supporters are eager to see new signings arrive - just as I am - but it would be wrong to sign a player when we are not completely satisfied with every aspect of the transfer.
"I can assure them that we are doing everything possible to improve the quality of the team before the season starts but rushing into anything won't do us any good."