'In last month's friendly against Northern Ireland, when we had Steve and Lewin in defence, our back four and the goalkeeper were given a severe test but they came through very well,' Toshack said.
'And against Cyprus, remember, we pushed Craig Morgan in there and he had a good 90 minutes. The big Cypriot centre-forward was found wanting.
'When these players come in, their concentration is high and we could get that from them again. I'm confident they would do a good job if called upon in these two matches.
'But we're hopeful James Collins will be OK to play. He played a reserve game last week, is due to play another one this week and could be in contention to play in the Premiership against Blackburn at the weekend.
'Out of the two, James is the more likely to be fit for Croke Park. Danny is a serious doubt for the Republic of Ireland and San Marino, but he was very keen to join up with the squad and see how the week goes.'
On the full back position, which will be between Ricketts and Duffy although Tosh seems a little disappointed in Duffy's progress. The two U21's are not ready yet.
'Richard did very well in his early games for Wales, but whereas some push on and do OK, others come to a bit of a halt,' Toshack said.
'The two young lads (Gunter and Eardley) who play in that position are coming along very nicely, and maybe they will have a shout in a year or so.
'Neal's played a lot more games than Chris. Chris has only had a couple of starts at Cardiff, but Neal's been a regular for Oldham in the pressure of a promotion campaign.'
On goalkeepers
'I'm told we've used four different keepers in the last four games - I didn't know we had four goalkeepers,' he quipped.
'In Danny and Darren, we've chosen the two experienced keepers for these two matches, while Paul has virtually accepted he'll be used mostly in a coaching capacity now.
'But we also have Wayne Hennessy making headlines at Stockport and Lewis Price playing regularly at Ipswich.
'I expect those two to push the two older keepers very hard for the position over the coming years. It's great to have such strong competition for places in such a key position.'