The 25-year-old, picked up on a free transfer from Wrexham, has a hamstring strain and has stayed behind for the club’s pre-season training camp in Spain.
Dalby had “looked sharp” according to head coach Steven Schumacher, but sustained the injury the day before the behind-closed-doors friendly against Tranmere Rovers.
In a further blow, Wil Forrester has also re-injured the hamstring which forced him to miss last season’s run-in.
In both cases, it is likely that they will be out until mid-August, with medical staff determined to take the recovery process carefully to avoid repeat injuries down the line.
“Will damaged his hamstring on day three of being back in,” Schumacher told The Bolton News. “He’s a week into the recovery now but obviously that’s not ideal.
“Then Sam Dalby got injured two days before we flew out here and he has a hamstring injury as well. He will be out for a little while too.
“It’s not the best. In pre-season you want everyone to be available because it sets you up for what is a really challenging season, you need to be as fit as you possibly can.
“There are ways now that we can get the lads up to speed but what is important, especially for those two, is that they are not rushed back.
“That’s what happens if you do rush them and it becomes a necessity, they break down, so we have to make sure that the rehab programme is spot on and they don’t have a reoccurrence during the season.”
In Dalby’s case, it is the first hamstring injury of his career but it does leave Wanderers light in the striker department as they go into their first pre-season friendly against Orlando Pirates on Friday.
David Abimbola has travelled with the senior team to Marbella, with John McAtee also likely to play as a central striker for the time being.
Wanderers are actively searching for at least one new striker but know they may need to wait until the end of the month to hear if they are successful in their bid to loan Celtic’s Johnny Kenny.