The Australian midfielder missed the 2-1 defeat against Liverpool with a hamstring issue, while the Englishman hasn't featured since February 10 due to a variety of niggles, including a virus and an ankle problem.
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"Darren has trained every day. He will be in the squad," Lambert told the club's official website.
"Darren had that month or so off when he couldn't do anything with his ankle. But he is back fit now and trained again this morning and is in the squad.
"Brett hasn't been getting in because Charles [N'Zogbia] has been playing extremely well.
"He wasn't fit for Liverpool. He is fit now. He has trained this week so he is back in the fray."
The Scottish boss was less confident of defender Ciaran Clark's involvement, however, after the 23-year-old picked up a shoulder injury while on international duty with the Republic of Ireland on March 26.
"Ciaran has still got a little bit of a problem," Lambert continued. "We'll see how he is the rest of the week.
"But he's still got a problem with his shoulder. He is still a doubt.