Mark Grew believes defender Clayton McDonald has proved his fitness to everyone at Port Vale in recent weeks.
The 22-year-old has started nine times for Vale this season, including the last three matches, during which time Micky Adams' side have been unbeaten.
"We do have to monitor his weight because he's a big lad," assistant manager Grew told BBC Radio Stoke.
"He's looking after himself and when you do that, it goes onto the pitch. His form's improving with every game."
McDonald was released by League One side Walsall at the end of last season and, because of worries regarding a long-term shoulder injury, was offered a six-month deal at Vale Park.
He has made 11 appearances in all this season and is likely to keep his place in the team when Northampton visit Vale Park on Friday.
"We knew Clayton was a solid defender and he's as honest as the day is long," added Grew. "He's a good communicator and great to have in the dressing room.
"He realised when he came that he may have to bide his time and he's done that. He's been very professional and worked hard. He deserves his place at the moment."