The Bray out-half came through the Leinster academy and made two appearances off the bench for them in the Pro12 two seasons ago but did not secure a senior contract.
He had a spell in New Zealand last year before coming back when offered a three-month contract by Munster earlier this season where his appearances were confined to the A side.
Now Lam has thrown him in at the deep end as Connacht bid to make it four wins in a row in their quest for a Champions Cup spot when they take on Zebre this evening.
Crosbie, who turned 24 a few weeks ago, starts at out-half where he will partner Ulster-bound John Cooney.
Lam has rotated his squad for this clash against a weakened Zebre. Connacht have won all eight previous league clashes with the Parma outfit, and have already claimed bonus point wins twice in Europe this season.
Danie Poolman, Niyi Adeolokun, and Bundee Aki are the only Connacht players to survive from the side which secured a bonus point win in Treviso on Sunday.
Forwards coach Jimmy Duffy says that having player of the season Aki back fit again is a huge boost as the champions look for a big finish to the campaign.
“With Bundee you know you are just going to get bundles of energy. But not alone that. He has actually got a very good strategic brain,” said Duffy.
“He can have quite a calming influence on the team. He sees and talks in such a specific way that actually organises people around him and gives them a lot of confidence.
“He has a huge amount of energy which we are all aware of and it’s really good to have him back. With these guys coming back, they have good experience and it helps lift the communication and quality of the session.”
Up front, Tom McCartney skippers a new pack where Eoghan Masterson makes his first start after being out with a knee injury picked up almost a year ago.
Zebre, who have lost their last eight league matches and not conceded fewer than 40 in their last four, have four of their Italian internationals back for this one but have had to draft two permit players from the Italian league.
D Poolman; N Adeolokun, B Aki, T Farrell, S Ili; S Crosbie, J Cooney; R Loughney, T McCartney, D Robertson-McCoy; L Stevenson, A Browne; E McKeon, N Dawai, E Masterson.
D Heffernan, D Buckley, F Bealham, Q Roux, J Muldoon, K Marmion, C Ronaldson, T O’Halloran.