The Republic of Ireland striker was sold to Hull City during the transfer window and has already scored as many goals for Steve Bruce's side as he had done for the Baggies this season.
And, after he was reminded of Long's abilities during West Brom's 2-0 defeat at Hull on Saturday, goalkeeper Foster admits he did not understand the decision to let him go at all.
"Sometimes I think why did we let him go to be honest with you," Foster told reporters. "It's not even hindsight.
"He was one of our best players from the last couple of years and it's disappointing that he's no longer here.
"He's a fantastic player and we would love to have him now.
"If you had asked the players, we would have liked to have seen him stay. It's a business at the end of the day so it's out of our hands."
Foster's disappointment is likely to stem from a personal angle as well as a collective one, as the 30-year-old's hopes of making this summer's World Cup squad will only be hindered by West Brom's poor form.
And, as the stats show, Foster is right to question the Baggies' decision.
Long has scored the most goals of any West Brom striker this season, while he had scored three in 15 appearances for West Brom before going on to double his tally for the season at Hull.
Whatever fee the club may have received for selling the 27-year-old, then, it will be nothing compared to the losses made if they get relegated.