Former England loose forward Westerman rejoined Hull last month after being released by the Canadian club.
He made his ‘second’ debut in Saturday’s 36-12 win over another former club Castleford Tigers and is included in the 19-man squad for Thursday’s Ladbrokes Challenge Cup sixth round tie at Championship part-timers Featherstone.
There was some confusion, though, as Westerman was named in the matchday 17 when Toronto faced Cumbrian amateurs Kells in an earlier round meaning he would be ineligible to now play for the holders in this season’s competition.
But Hull - ravaged by injuries - put their case forward to the RFL and Radford explained: “I knew he didn’t take to the field.
“He warmed up and pulled out even though he was named on the bench.
“We didn’t find out until Tuesday whether he could play and the RFL had to watch the game back to make sure he didn’t get on the field.
“I took Joe’s word for it and he had pulled out in the warm-up.
“It’s huge for us if I’m honest. It again magnifies the importance of bringing him in over the last three weeks. He played a huge part on Saturday and now we have another body for Thursday as well.”
Hull need it given they are without three half-backs for the trip to the Championship high-fliers.
Marc Sneyd suffered a knee injury against Castleford joining Albert Kelly and Jordan Abdull on the sidelines.
Initial hopes were Sneyd could be back inside six weeks but Radford today admitted the back-to-back Lance Todd Trophy winner could even be out for the season if surgery is required.
“There’s a chance,” he said, when asked.
“We will go and see a specialist moving forward.”
Radford also admitted he is unlikely to enter the loan market for any new players to ease their injury crisis.
“There is scope (salary cap) but what’s out there is minimal,” he said.
“We obviously have no quota spots and we would be looking at a UK-based player and there aren’t a great deal kicking about at the minute.
“There are an awful lot of squads in a similar position to ourselves in terms of injuries. The fact that Hull KR has been looking for five or six months, Widnes have been active in looking for loan players and I have had phone calls off Wakefield as well.
“There are a lot of teams busted up at this stage of the season which obviously reduces the amount of players you can bring in.”
Radford will field centres Jake Connor and Carlos Tuimavave as his half-back pairing versus Featherstone as the clubs meet in the Challenge Cup for the first time since the West Yorkshire club’s shock Wembley win over the Black and Whites in 1983.
“It’s two centres playing in the halves but they are still fair players and would still get in most Super League sides’ half-back roles,” he said.
“But what it does do is thins us in the centres unfortunately. We are low on numbers but we still have a great 17 out.”
Westerman, 28, has acted as a makeshift stand-off earlier in his career but won’t be used in that position against Featherstone.
“He thinks he’s an option in the halves,” smiled Radford.
“The role he plays for us he is very much suited to. He has enough grunt to go forward but he also links really well between the middle and the edge.
“He does a really good job of holding their middle up which creates some space for our outside backs and makes them look a little bit prettier.
“He looked like he hadn’t been away on Saturday when he fell back into that role.”