Alessandra, out of contract this summer, initially agreed to put pen to paper on a fresh deal to remain at the Globe Arena but has now instead taken up the offer of a two-year contract with the Pilgrims.
Bentley told the club's official website: "Lewis has done really well for us over the past two years and I am obviously hugely disappointed to lose him.
"We had a chat a few weeks ago and he said he intended to sign on again but has since had a change of heart with another club coming in for him.
"Although I am disappointed it means we will have to look at other avenues and stay positive. I have been talking to a lot of players in recent weeks and will keep chasing a number of targets we have in mind."