Meanwhile, striker Fejiri Okenabirhie will miss at least the next eight weeks with a foot issue.
New signing Hiwula suffered the injury during the friendly against Sheffield United, which forced him off at half time and is likely to miss the first three months of the campaign at least.
“He is going to be out for quite a considerable amount of time,” Wellens said.
“He got a bad tackle against Sheffield United with about two or three minutes of the half to go. He carried on but we’ve had it scanned and it’s not good.”
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The former Coventry City and Portsmouth forward joined Rovers three weeks ago but faces a lengthy wait for his first competitive minutes.
Okenabirhie carried a foot injury into pre-season but suffered a setback after returning to training last week and is set to miss the first two months of the season,
“Fej will be at least eight weeks,” Wellens said.
“It is what it is, we crack on.”
John Bostock returned to training on Sunday after a longer than expected recovery from Covid-19.
Ben Close is set to report back for training on Wednesday after completing his recovery from coronavirus.
The midfielder will feature in a behind-closed-doors game against Watford at St George’s Park on Wednesday as Rovers put together their final preparations for the season, which kicks off against AFC Wimbledon on Saturday.